Credits: This guide was created by Th3Simon (a player of Darkness Reborn). If anyone deserves credits, it should be him.
Caution: Wall of text ahead.
1. Introduction to Warrior class.
Long spear, huge sheild and pack of muscles. What else do you need to slap them monsters in face? Warrior is a very good pick for new players and experienced ones. It's variety of skill effects allows you to build him in aggresive or defensive way, while HP pool is the highest of all 3 classes, regardless of the build. Defensive one provides you with high armor and survivability making you difficult to kill in PvP and helping you get through tough levels or bossfights. Aggresive one lets you wipe those levels or rifts in matter of no time or destroy those squishy Kunochis if played properly. And if you feel you like neither of those above, you may balance him equally between ATK/DEF but in my opinion its better to excel in one thing and be weak in other than to be mediocre and everything and never shine.
2. Skills in general.
Short info about all skills available for Warrior. More info included in Skill Recommendations. I didn't include percentage/damage values as it is visible ingame. Posted only info which may be not so clear from ingame descriptions. As "Levels" I also mean rifts
RUSH
Typical Dash including one hit. Lets you avoid all attacks and skills while active.
Available options: Physical, Physical with speed boost, Lightning.
A VERY useful skill in almost every part of game. Lets you escape from hard situations in PvP or finish off enemy that is running away.
SHIELD COUNTER ATTACK
Hold button to use shield. Works for a limited time(depending on level of skill). Avoids all attacks and skills from all around, while holding button. If used just before attack, lands a counter dealing damage in front of you.
Available options: Physical, Enhanced Physical, Lightning
Skill mainly for boss raids because of duration time and short cooldown. May be useful in PvP or PvM.
SPINNING ATTACK
Spin with your spear, pushing away enemies around you, dealing damage. Activates super armor state (less damage recieved while active).
Available options: Physical, Ice with speed debuff for enemies, Enhanced Physical
Very good skill to wipe levels quickly or for kiting masses of enemies. Not so useful in Boss Raid, quite good for PvP, especially Ice option with slowdown for escapes.
SHIELD PUSH
Hit enemies in line in front of you with shield, followed up by 2nd hit with effect dependant on chosen option.
Available options: Physical, Enhanced Physical with stun, Fire with burning effect.
Excellent skill, especially with stun, for all game modes. Deals massive damage. Good for engaging enemies and escaping.
STAB
Hit quickly dealing low dmg, or hold button to charge and deal massive damage.
Available options: Physical, Physical with bleeding*, Ice with speed debuff
Most useless skill in my opinion. Charges too long while dealing not so much damage. Easy to predict in PvP so its easy to avoid + cooldown is horribly long. Can be useful for wiping levels, but other skill is way better.
*I don't quite understand bleeding effect. I think it's kind of physical damage over time, affected by armor, similar to burning or poison.
PATIENCE
Turns on shield that buffs your/your teammates armor or buffs your attack for duration time.
Available options: Self-shield, Attack buff, Self/Team shield
Self shield is best all-round skill for all game modes. Can help you survive extremely difficult 3v1 PvP or Bossfights. Attack option good for quickly wiping levels, TeamShield is mediocre, but may help in PvP.
ULTIMATE SKILLS
No need to describe this I think. Pick whatever you like and whatever deals most damage at the moment. Area of effect is the same, Damage is similar depending on Level. I would recommend picking the ones that suit Your build*
3. How do i build myself?
Few words before I begin: by the definition of build i mean jewels that you put into items, picked skills, and finally passives.
I did not make a "Passive skills" section because they all are well explained ingame, and there is nothing unclear about them.
First and most important: DPS + ARMOR + HP - Those are the main attributes you want to focus on, regardless of additional options included. But don't go too hardcore! If you have, for instance, Weapon (all are non enhanced) with 2000 ATK and +2% Lightning(which is your main) and weapon with 2010 ATK +2% Poison (which is useless for Warrior) do not pick the second one.
Second and also important: If you have equipment that you ll use for 4-5 levels(Rare/Epic/Legend), enhance ENHANCE ENHANCE! Always enhance, because it's cheap and efficient way to maximize stats.
Third: If you earn enough SOL or wanna spend money, ALWAYS max your MP skill passive whenever possible. Earlier on i was going easy on it and found myself constantly running out of mana which makes you extremely vulnerable on harder levels and tougher PvP matches. Now that i have 10lvl MP passive, 10lvl MP Regen Passive + nice ring adding max MP i rarely run out of mana.
I think i don't have to mention HP and Weapon ATK passive which is obligatory to max as soon as possible
So how do you build Warrior?
First of all try to pick all skills with similar elemental effects.
Example:
Rush: Lightning,
Shield Push: Physical Stun
Patience: Anyone
Spear Expert : Lightning
2nd Example:
Rush: Lightning
Spinning Attack: Ice
Stab: Ice
Javelin: Ice
Main goal of it is to focus on one, max two element's and match passives and equipment with them.
Let's consider second option:
We want to build Ice, so things you want to do to be most efficient is:
Maximize regular passives: Weapon ATK, Armor, MP, HP, Critical Rate, Critical Damage
than maximize your Ice passive.
Next thing is to put jewel that boosts Ice attack to your weapon.
Third is to try to look for equipment that boosts Ice ATK as their one of attributes.
That way your abilities will deal much more damage and maximize your efficiency on battlefield.
You might think that "Hey but according to stats Physical Stab deals more than Ice stab!" Answer is that after adding those % from jewels/equipment it evens it out, or even makes you deal more damage. Anyways feel free to experiment 
4. Strategy & Skills in PvM.
You might think that it is simple as that and needs no explanation, but few tips may increase your efficiency on wiping levels and maybe it will help you beat tougher ones.
Just to be clear PvM is Player vs Monster(s)
Best way to clear levels is to kite. By kiting I mean running around level so that monsters follow you and group up in dozens. That way when you turn around and hit you deal damage to almost all of them. Same goes for skills.
What is the best skill set?
There are 2 answers depending if you are trying to beat hard level, or just need quick exp and money so time matters. Ultimate, as mentioned earlier, is up to your build and preferences thus pick any of them.
Hard set:
Rush/Shield: depends on build -Rush lets you escape from packs of enemies or avoid skillshots of bosses. Better than shield in case you get cornered but tons of monsters and need escape. Shield Counter Attack is useful when fighting bigger bosses such as Orc General on Hard which attacks quite fast. Its better for levels where you have less small monsters and more semi-bosses, stage bosses.
Shield Push: stun - Deals more damage than spinning attack + stuns enemies which sometimes may give you few more seconds to escape or to wait for ultimate skill to go from cooldown. Also it interrupts attacks from most units except main bosses.
Patience: shield - This is the BEST all round skill and I will mention it many times more, because it more than halves the damage you recieve of ANY type. Boss attacks will deal 300 damage instead of 1000 which is HUGE difference. It also has low mana costand long duration which makes that skill a MUST HAVE in tougher levels.
Easy/Quick set:
Rush: any(Lightning recommended) - Same as above + it lets you move quicker and deals quite nice damage.
Spinning Attack/Shield Push - Both are good for wiping levels, Spinning is good when you rush into group, Shield Push is better if you kite.
Patience: Attack - Self explainatory. More damage = faster killing.
Later on I plan on tutorial on how to beat certain bosses without super-duper equipment. That's all of it for now.
5. Equipment
There are 5 kinds of equipment:
Grey(Poor), Green(Common), Blue(Rare), Violet(Epic), Orange(Legendary)
Grey to Blue are obtainable from levels. Blue as unidentified mostly, from bosses such as Shelob.
Violet as a reward from Boss Raids or Rifts(Very rare), from workshop crafting (easiest and most certain way) and strongboxes.
Legendary are only available from strongboxes and workshop crafting. No monsters drop them afaik.
Crafting materials described later, in Rifts section.
GS means level of Weapon. It starts from GS1 up to GS55. Higher the level, better the weapon. Simple as that. There is minor difference between normal items, and those with [A] or [S]. [A] is slightly stronger than normal, while [S] is stronger than [A].
Important things are as mentioned earlier: Always Enhance if you plan on keeping equipment for longer than 1-2 levels.
Priority is in that order:
Weapon -> than Armor
Jewels:
Jewels work as a swapable bonus for your equpment. If you put Jewel into Weapon, it will acquire ATK bonus. Put it into Armor, it will acquire Resist/Armor bonus. It never gives both at one time.
Topaz(yellow) - Lightning ATK/Resist
Sapphire(turquoise) - Ice ATK/Resist
Ruby(orange) - Fire ATK/Resist
*need name*(greenish) - Poison ATK/Resist
Zircon(blue) - VS Mode ATK/Armor
Garnet(red) - Weapon ATK/Armor
As Warrior try to avoid poison Jewels in weapons as this class has none skills that use this element.
Focus on your build mainly: +Weapon ATK Jewels, Crit and all other that max out your damage.
If you fancy playing PvP a lot try VS Mode jewels.
For Armor, Diamond Jewels are good as they are for all element resist.
If you are still developing your toon that info may be useful:
Archen Forest minions deal mostly Poison damage
Jacculen Mine minions deal Fire/Ice damage
Fallen Iron Castle Fire damage + i need to find out the rest 
6. Rifts
Each time you finish any level on Hard mode, there is a chance (somewhere between 20-35%) that a Rift will apear. It's a small 3-stage fight with stage bosses and sometimes final bosses with weaker stats. Mobs appear accordingly to Act you're in (Forest, Mine, Castle)
They are main source of money and crafting material. POOR is obtainable in all Acts. COMMON is available from Harder Forest Rifts(with Main Bosses) and then drops more often higher the level of Rift, also available as general PvP reward. UNCOMMON starts dropping in Jacculen Mine rifts and is also available as PvP ranking reward. SUPERIOR is the rarest one and drops from Rift with Damien as Stage 3 Boss and as a PvP top 100 reward at the moment.
I would suggest to play more rifts from level 25+ as it is the time you should start PvP'ing more and try to craft better items. For levels 1-25 it is kinda waste of energy since gold drop is quite small, and 90% of the time you will acquire only POOR crafting material. Grinding is more important for early levels. Untill then you dont need any crafting because you level so fast that equipment will be changed often thus all crafting material and gold will go to waste.
Strategy is a no brainer, just kite and kill them all
7. Where to gain exp. and how to level up quickly?
Experience is ONLY gained from campaign Normal and Hard mode. Rifts doesn't give exp, so does PvP and Boss Raids.
Thanks to patch 1.0.7 from now we can see how much XP we gain from beating certain levels. Try always to pick up the levels that pay off the most at the moment. For the while, the best level to grind, is stage 14 from Jacculen Mine. On Hard mode (even if its greyed out) it gives as much as 27k exp, and it can be more if you have equipment with experience bonus and passive with that. Even level 10 from Fallen Iron Castle does not pay that much, which i find weird. Maybe in future it will be fixed or changed.
If you read carefully you see that there is bonus experience equipment. If you only grind, I'd suggest to keep equipment that has bonus exp just for the moment that you beat damien and want to level up to 60 quickly. Since level 14 of Jacculen Mine is super-easy to beat from levels 35+, there should be no problem with downgrading for a little while from epic GS50 Spear to GS35 Spear with +2.5% bonus exp.
Also maxing out the passive for bonus exp is a good idea, but I would do that for sol in later stage of game.
General recipe to grind is:
Get 30 friends
Send points so you have 300 "hearts" to spend on 30 energy.
Login from time to time to recieve bonus energy as a reward
If you run out of energy play some PvP since it rewards with energy sometimes.
Play level 14 of Jacculen Mine or the one that pays the most accordingly as much as you can. Till your thumbs bleed 
8. Getting yourself rich in Gold and Diamonds
Spin the roulette!
Do daily quests. They are usually quite easy to do and result in Gold and SOL so it is very profitable for you. After you run out of quests and achievements do this:
Find a rift. It usually appears every 3-4 levels beaten on hard. Later on play rifts and rifts only untill you run out of energy and hearts (friend points). If you feel like you cant beat rift alone, do a Random Invite. Dont wait those 15 seconds, just press invite and cancel, invite and cancel and so on untill someone appears. Cancelling doesnt remove the invitation for users. But doing it certainly saves your time.
To save energy from searching for rifts just wait in lobby. Invites usually apear every 1-2 minutes.
15 minutes of playing rifts usually gives about 15-25k gold depending on level.
After you run out of all energy and hearts do PvP. Wining or losing always give you some gold.
And what about them diamonds?
Daily quests, Achievements - First source of SOL
Playing PvAI (PvP against AI) gives 10 per day or even more if you beat easy and medium a few times
PvP is the thing that pays off the most. Being in first 10% gives you 75 SOL per week which is huge amount. + one or two SOL's per every 2nd or third match
Boss Raid sometimes pays off with SOL but imo it uses too much energy + the ranking doesn't pay with them.
Spin the roulette!
9. Fighting and Skills in PvP
So you wanna climb the ladder and become one of top 100? Me too and im closer and closer so here are some tips and tricks for PvP mode.
Most important thing is your equipment and passives.
After you earn enough SOL max out your MP Passive, Weapon ATK, Critical Rate/DMG and all the ATK/HP passives + your build passive (ice/lightning recommended).
Try to craft Epic equipment from workshop and enhance it to the maximum.
Suggested level to play PvP is between 25-35 as most people out there dont have many legendaries yet and gameplay is quite balanced. (Despite all lags and teleports and other buggy stuff)
After your toon is packed with nifty shields and armors, glowing with passives, its time to kick ass.
But what skills are best for me?
This is the set I have been personally using and I find it the most efficient and handy for all PvP battles.
Skills:
Rush: Lightning - My main build is for lightning damage. Also it almost always lets you escape from tight situations like 3v1 or to jump for Warlord Orb. It is a good kill finisher, if you see that your enemy is quite low on HP just dash through him for a quicker kill. Shield counter ****s, because you cant move and do anything. It also has to be timed well to do the counter part.
Shield Push: Stun - Combined with rush it does a nice combo. Good for dealing damage and for escaping. The stun gives you many opportunities to pick the Warlord first, or healing orb, to run away. Also damage is good.
Patience: Shield - A MUST HAVE! Better than Teamshield because it costs less mana and lasts for longer. Helps you survive almost any teambattle or teamrape. Combined with Armor Orb makes you invulnerable for a while. Its better than Attack buff because you mainly hit with skills, you rarely have occasion to hit with your spear many consecutive times. And shield can be activated even while you are being hit which gives durability.
Orbs:
Warlord Orb - Most important orb ingame as it lets you use your Ultimate Skill one time. Very good for teamfights, may win the whole battle. Spawns on 2:35 on right side of map, and respawns every 25-26 seconds.
Only Warlord spawns on right. All other orbs spawn in middle or left side of the map.
First pop time is from 2:35-2:32 and than every 20-25 seconds after it was picked up. Middle seems to pop orbs once per 20 seconds when time is almost over.
Armor Orb - Works like Patience: Shield. I dont know exact values but i think it multiplies your armor by 3. With active Shield it makes you almost invulnerable. Effect Duration 30 seconds.
Health Orb(potion) - Works just like normal potion. Spawns in Middle or on Left side of map Spawning time to determine*. Spawns as first orb on left side. Later on its random.
Mana Orb(potion) - Same as above. Spawns randomly,
Speed Orb - Speed up your hero, but only the movement speed. Effect Duration 30 seconds.
Power Orb - Works similar to Patience: Attack Powers up your skills and attacks. Effect Duration 30 seconds.
Important info is that Orb's doesn't stack. If you have Warlord, be careful not to pick up any other or you will lose it.
Strategy:
Main goal of PvP is to kill as many times as you can. To do that you need to focus your attack on one player. Due to lack of general voicechat of any type of communication during the battle you have to play instinctively. Follow your teammates, try not to split as with force of 3 you are much stronger.
After the battle begins, run for Warlord Orb. It is on the right side of map at the end of "hallway". It spawns exactly on 2:35. Keeping that part of map lets you control other team as you can easily crush them once per 25 seconds with your ultimate. Remember to always turn on your Shield!
Best way to fight is to use combo: Rush, followed by Shield Push and if you stun them succesfully land a few hits to soak up damage. When you are low on HP head to left side of map to grab armor orb or HP potion if lucky. Remember about Shield! To escape from losing teamfight, Rush out and Shield push them to stun, than run away. Keep an eye on Your and Enemies health bars so you know whats going on.
More info about PvP against specific class soon.
10. Fighing & Skills in Boss Raids
Boss Raids are recommended from level 30+ IMO. Level 25 is just too low, the equipment is not enough and skills are not developed yet.
Remember that you can choose which Belphegor level you can fight. Only levels 1-2 are recommended. Level 3 is only when you are high level (55-60) and your teammates are similar.
Skills:
Shield Counter Attack - Pick the one that lasts the longest. Better skill than Rush because the "Evade" part lasts much longer which works if you dont time the dash quite well. Plus it compensates eventual lag. Useful to avoid Belphegor's attacks
Shield Push - Deals more damage than Spinning attack, thats all.
Patience: Shield - Use it all the time as it reduces damage.
Strategy:
For Gargoyles, just stay at their back and watch if they start to swing their weapon or fly high. If yes just use Shield Counter attack and than just hit them hard.
Belphegor in 1st stage is quite easy. First, go to one of the corners, the closest one preferably. Than watch if it mouth glows. If yes, than turn on Shield Counter Attack to avoid DragonBreath. Also look if he swings its arm. If yes, repeat as earlier.
Belphegor in 2nd stage is little more difficult. Try to stay at his side/back all the time and hit him hard. When he flies up use Counter and wait because it is quite long attack. Than repeat it all the time. Avoid staying in front of him as sometimes he rushes forward dealing massive damage.
That's all. Ain't that difficult, but its definately timetaking.