Tips on Upgrading and Fusion

The trick to be the most successful at upgrading and fusion is essentially the ability to min-max at every point of upgrade. You should try to minimize the risks and maximize the value and chance of each upgrade attempt. It may seem complicated, but it is very simple. In the long run, this will save you both time and gold spent on upgrades and fusion.

So lets take a look at the details:
When upgrading, the chance of success is based solely on the difference of the heroes' rarity.
Difference = Rarity of Upgrading Hero - Rarity of Sacrifice Hero

For example, here are the chances of upgrading for a 5★ hero:

Rarity of Sacrifice Hero	Difference in Rarity	Chance of Success
5★	0★	100%
4★	1★	50%
3★	2★	25%
2★	3★	10%
1★	4★	1%

Note: A rarity difference of 5★, when upgrading a 6★ with a 1★, has a 0.1% chance of success.

As you can see, upgrading a hero with a hero of the same rarity will always result in a success. Similarly, using a hero of one lower rarity will always have a 50% chance of success, and two levels lower will result in a 25% chance of success.

For example:

 • Upgrading a 4★ with a 3★ will have a 50% chance of success, rarity difference of 1★.

 • Upgrading a 3★ with a 2★ will have a 50% chance of success, rarity difference of 1★.

 • Upgrading a 3★ with a 1★ will have a 25% chance of success, rarity difference of 2★.

What we want to do is to fully upgrade a hero while using the least valuable heroes as possible, as higher rarity heroes are way more valuable than gold. Using five 6★ heroes to fully upgrade one 6★ would be ridiculous, as each 6★ used is more valuable than the gold saved.)

Let's take a look at the chances of success according to the difference in rarity.
From a difference of zero to one, the chance of success is halved from 100% to 50%.
And from the difference of one to two, the chance of success is halved again from 50% to 25%.
However, from the difference of two to three, the chances of success is more than halved, from 25% to 10%.
And from a rarity difference of three to four stars, the chance is decreased even more by a whole magnitude from 10% to 1%.

Conclusion: In order to maximize the chances of success and value of the sacrifice hero, a good rule of thumb would be to always try and upgrade a hero with another hero within a rarity difference of 2★.

However, let's say that you have two 4★ level 30 that you want to fuse together. That would mean that you need to have 10 successful upgrades before you can use them in fusion. Finding a bunch of 2★ to use in upgrades is not easy. At this point, if you are sitting on a bank of gold, you may consider using many 1★ to upgrade. While not ideal, they do still give a 10% chance of success when upgrading a 4★, and they can easily be found when farming in Keronic Base. Farming 7-1 often gives you a 1★ Ghost while 7-9 gives you a 1★ Vulcan. So you might consider using a bunch of 1★ heroes as fodder to upgrade a 4★ up to +1 or +2, while the cost is still low.

However you decide to play, it is up to you to balance your gold on hand with your hero upgrades.