First thing required before upgrading heroes — a hero called "Warlord". You will obtain him after reaching account level 49. This guy increases experience gained when leveling cards by 33-64% (in other words, he reduces ammount of cards you normally need by half). This results in great reduction of helmets needed to purchase hero packs. I advise to stop leveling him somewhere at lvl 55, since any further upgrades will require way too much helmets for evolution.

How you should not upgrade heroes
Previously i was opening hero packs to get many lvl 1 cards and simply sacrificing them to a single hero. Later found out this is a huge waste of resources. There are few improtant rules you must know before diving deep into math.



1) Sacrificing hero card to a card of same type grants a lot more experience. For example, your lvl 1 Warlord can consume other lvl 1 Warlord to become lvl 10, but consuming anything else will improve him only to lvl 6. Difference is huge.

2) Warlord must be equipped in order to get experience bonus.
(All math i am gonna use includes equipped warlord 55 lvl with 6/6 charges)

3) Lvl 1 card gains a lot more benefits from "eating" big cards, unlike other way around. Here is example: you have 10 lvl Warlord and 1 lvl Warlord. If you gonna sacrifice lvl 1 card to a big one, his level will increase a bit 10 -> 14... But if you going to sacrifice big card to lvl 1, a funny twist gonna happen: 1 -> 16. This happened because of rule №1. Since bigger card had more experience inside, it gave even more in return when feeding to card of same type.

4) When sacrificing high-lvl hero to same hero type (1 lvl), you will get +6 extra levels.

5) Red heroes have 90 lvl max cap, which makes them perfect feeders.


Alrigh, now let's take better and bigger example of how you can utiliza rule №3. Let's say, you have 6 cards of the same type. We are going to take card lvl 1 and feed it another lvl 1 to get result "10 lvl card" — we will call it "feeder". Then take fresh lvl 1 card and feed it with our feeder. Continue doing this until card reaches max lvl.

• (step 1) 1 eats 1 = 10 lvl feeder;
  
• (step 2) 1 eats 10 = 16 lvl feeder;
  
• (step 3) 1 eats 16 = 22 lvl feeder;
  
• (step 4) 1 eats 22 = 27 lvl feeder;
  
• (step 5) 1 eats 27 = 30 lvl feeder (maxed).

This process called "feeding", it helps to create 30 lvl green cards very fast, while using little to no resources. Let's compare numbers:

Normally you need 41 cards to improve green hero from lvl 1 to 30.
When eating cards of same type, you need 14 to improve from 1 to 30.
By feeding you need only 6 cards of same type to improve from 1 to 30.



Formula above requires 5 steps to max out green hero. That's a lot of clicking and scrolling around, which is annoying and long process. So here is a way to shorten it, by using same ammount of cards:

• (step 1) 1 eats 1 and 1 = 14 lvl feeder;
  
• (step 2) 1 eats 14 = 20 lvl feeder;
  
• (step 3) 1 eats 20 = 25 lvl feeder;
  
• (step 4) 1 eats 25 = 30 lvl feeder (maxed).

Conclusion
As you can see, feeding system allows to get lvl 30 green heroes by using just 6 cards. Major problem is max cap of 30 lvl. To breach it you need helmets for hero evolution. More over, you can't simply feed evolved card to lvl 1 card, since lvl 1 card will still have hard cap (30 lvl max).

This is the moment we need to think about other colours (ranks), since they have higher hard cap, which is much better compared to green. But there is new problem — blue, purple and orange hero parts are much harder to get, you can't simply feed blue guys to other blue guys, this will require way too much cards. Yet expenses can be cut!



Let's take a blue hero with 45 max level cap:

Normal process (bad):
(step 1) 1 eats 1 = 14 lvl blue;
(step 2) 1 eats 14 = 21 lvl blue;
(step 3) 1 eats 21 = 28 lvl blue;
(step 4) 1 eats 28 = 33 lvl blue;
(step 5) 1 eats 33 = 39 lvl blue;
(step 6) 1 eats 39 = 45 lvl feeder (maxed). We spent 7 blue cards to get there = too many, we can't afford this.

Double feed:
(step 1) blue 1 eats green 30 = 28 lvl blue;
(step 2) blue 1 eats blue 28 = 33 lvl blue;
(step 3) blue 1 eats blue 33 = 39 lvl blue;
(step 4) blue 1 eats blue 39 = 45 lvl blue feeder (maxed). We spent only 4 blue cards to get there.

Triple feed:
(step 1) blue 1 eats green 30 = 28 lvl blue;
(step 2) separate blue 1 eats green 30 = 28 lvl blue;
(step 3) blue 28 eats blue 28 = 39 lvl blue;
(step 4) blue 1 eats blue 39 = 45 lvl blue feeder (maxed). We spent just 3 blue cards to get there.

To optimise triple feed i suggest combining step 2 and 3, so it will look like this:
(step 2) blue 1 eats green 30 and blue 28 = 39 lvl blue (1 less steps this way).



Purple cards have 60 lvl max cap:

(step 1) purple 1 eats blue 45 = 43 lvl purple;
(step 2) separate purple 1 eats blue 45 = 43 lvl purple;
(step 3) purple 43 eats purple 43 = 54 lvl purple;
(step 4) purple 1 eats purple 54 = 60 lvl purple feeder (maxed). We spent just 3 purple cards to get there.

Optimisation can be done by combining steps 2 and 3:
(step 2) purple 1 eats blue 45 and purple 43 = 54 lvl purple (1 less steps).



Now let's take a look at orange hero and how he will respond to different feeders colours.

1) If lvl 1 orange eats 60 lvl purple, he will become 57 lvl.
2) If lvl 1 orange eats two blue cards 45 lvl, he becomes 51 lvl.
3) If lvl 1 orange eats 4 green cards 30 lvl, he becomes 42 lvl.

To summ it up: eating higher level feeders is better. Now question — maybe you should simply create many-many green feeders and just feed that orange? How efficient is this in terms of steps? Take a look at this:

• Making one maxed green feeder takes 4 steps;
  
• Making one maxed blue feeder takes 12 steps;
  
• Making one maxed purple feeder takes 28 steps.

In order to reach efficieny of purple feeder and allow our orange card to get lvl 57, we will need 14 green cards of 30 lvl. And that is 56 steps! Instead of wasting double time to get same result, you should make two purples at 60 lvl.



....

28 november 2019: guide was created;
29 november: added few pictures and corrected some text;
8 december: added more pictures and +2 rules for clarity.