This guide will answer your questions about best way to progress to maximize the use of gold and sol (assuming you are a free / minimal spend player like myself) on Darkness Reborn for iOs and Android.
This is not intended for paying players (purchase SOL regularly).
Energy Utilisation
Let's say you have maxed your friend list of 30, this means every day you can send 30 hearts = 300 friend points = 30 energy
If your friends are good they will return the favor and you will receive another 30 energy
Daily log in is free 20 energy
Daily event (does this happen every day?) is 10 energy and 5 vs mode tickets
Every now and then if you win a VS mode game you will be rewarded 1-2 energy, assuming you win 5-10 games a day this averages out to be around 5 energy
so maximum free daily energy = 95.
Then there is the normal timed energy capped at 25, 10 minutes per energy. Meaning 6 per hour and 144 per day max.
Assuming a normal player (that doesnt play 20 hours a day) you will get around 150 - 180 energy a day.
So about 50 Hard mode or 80 normal mode.
Level 1 - 25
Should be able to do this in a couple of days if playing regulraly. Focus all energy on leveling, no need for rift / boss raid or anything else. Should be pretty straight forward as the quests are not overly difficult.
Exception: if it's end of week for VS mode or Boss raid, do one of each to get the baseline rewards.
Level 25 - 40
This is where you need to be tactful on how to spend energy, try to grind hard mode level with lots of mobs for exp and save up equipment, you need to make some friends as if you spawn a level 30+ rift you should do a few of those as well just to collect some materials.
Depending on the state of your gear, you may want to grind a few levels ahead of boss levels before attempting them - it saves you energy in the long run. However if you have gotten lucky with drops then just continue - you can gauge this by how quickly you can clear the previous level. If you have to use pots, then most likely you cant tackle the next boss easily.
Level 40+
At this point you should do any rift when possible, try to maintain at least above 100 rank in boss raid and VS mode. This will ensure maximum rewards with minimal effort, rift will provide gold for learning out skills - I have my main passives on level 5 and sub skills 2-4 depending on how useful it is. With remaining energy grind the highest level Hard mode you can manage with minimal pots - this will ensure you get plenty of items to use, sell or as enchance material. Tackle each level slowly as you cant really rush anymore unless you have epic / legendary weapon.
Next topic is SOL usage:
There are a few ways to do this, one is to save till 40 - 45 and buy a few boxes there to help with grinding. At the point where you struggle to even complete the lowest green hrad mode without pots, you should consider buying a few boxes with the SOL you have saved up. Of course dont spend all of it.
Priority of items is weapon > armor > accessories
The second option is to save ALL your SOL until you are max level, this will ensure you do not waste any, all of the items you get will be retained (until cap lift, which will probably be in a month or two at least). This way it will take much longer to reach cap, but you will end up with possibly lot more SOL to buy items at that point. This really depends on how lucky you are, what items you get and how much they will help you in boss raids / vs mode to earn more SOL or items.
The same rules apply to crafting. Do not craft unless it's at least epic and only if it's a massive upgrade on weapon (less inclined to build armor).
However, try to craft level 6 or above garnet if you see it, reason is it will help with grinding on a high level weapon.
Finally, when to open strong boxes
The game gives you various rewards with free boxes and such, if you are not struggling to grind then should save them for as long as you can - you will get higher level items.
If you can save some into level 40s, they will definitely be more worthwhile. For earlier levels quest rewards should be quite sufficient already, there is no real need to buy/use boxes.
Hopefully that'll help you get to 40+ with the most resources saved, feel free to post any questions.
Credits: Special thanks to Aides for making this informative guide.