DrawFighters (EP)

All you need to do is buy a total of 20 different skills between all your fighters (if you buy the same skill on two different fighters, it only counts as 1). As you advance into the different volcano arenas, you will unlock new fighters. I would choose the lowest leveled, weakest ones that you aren't planning on using at all during the game and just buy the cheapest skills you can with them until they run out of skill gems. Once you have a total of 20 different skills unlocked across all your fighters, the trophy will pop.