When you level a hero up, the EXP required increases slightly each level. However, this isn't always a steady increase. At higher levels it becomes exponential, with the amount of EXP required at each level increasing more and more. By looking at how EXP varies we can work which method of evolving heroes is the most efficient for both hero aids and main heroes.
Firstly let's remind ourselves when heroes gain an increase in aid strength:
In short heroes gain aid strength every 10 levels until 180, and every 5 levels for levels 180-200.
You can tell just by looking at the EXP difference every 10 levels that the lower level hero aids are the best to work on, as these will level up in aid strength the fastest.
Here's how the EXP requirement for each level varies for all heroes:
From this we know that levels:
1-110: A relatively steady increase from 20 to 9590 EXP per level
110-180: Levels remain a steady increase between every 10th level, where there is a large increase which rises in a slow exponential manner.
180-200: Rapid and continuous exponential increase that constitutes over 50% of all EXP required to fully upgrade hero.
From this we can conclude that the best way to go about evolving heroes is to always improve the lowest level hero first, as this will result in the quickest hero aid improvement.
Also, try and make the increases in batches of 10 if you can. There is no point in upgrading a hero from level 150-155 (which will only get you halfway to increasing their aid strength) when you could've used that same EXP to upgrade a hero from 120-130, which will increase their aid strength (apart from the actual HP and ATK increases of course, but this only matters if they are used as main heroes).
Here's a handy gif example to illustrate how this means heroes should be levelled:
(Note that aid slots are irrelevant here!)
This also means that once a hero reaches level 180, it's going to take a much longer time to increase their aid strength, so you should once again start improving lower level aids instead of pushing beyond this.
Now here's a second chart showing the TOTAL EXP fused into a hero at any level:
This graph shows just what fraction of EXP you've fused into them of the total EXP your hero needs to hit level 200. Each vertical slice of the highlighted area is equal, and represents 10% of the EXP your hero needs to hit 200
When a hero hits level 126, you've fused about 10% of all the EXP they'll need. By level 153 you've fused 20%. In fact, by level 187, you've only fused 50%. Yep, you read that right. By level 187 you are halfway complete in levelling up your hero!
All in all it takes 6,091,810 EXP to fully level any evolvable epic hero. That's equal to 812 Coral Eggs.
To level a rare hero to level 160, you'll need 1,447,340 EXP, approximately 25% of the EXP it takes to fully level an epic hero, and equal to 193 Coral Eggs.
1.) Don't go above level 180 for hero aids, until every single one is at level 180.
The HP and ATK aid increases are very small beyond 180 and they take an exponentially larger amount of EXP to reach.
(Ignore this rule if you enjoy spending inordinate amounts of time working towards virtually nothing)
2.) Work on your lowest hero aid first, as this will give the fastest aid strength increase.
Lower hero levels require less EXP to reach, and so will lead to faster HP and ATK aid increases.
*Obviously these rules don't apply if you have a specific goal, or need to get a hero to a higher level for whatever reason! These are just the default guidelines for the most efficient overall levelling method.
Hope this helps! Let me know of any amazingly embarrassing or disastrous mistakes that could potentially discredit this entire thread.