Yea there isnt a tutorial. But its simple: You gain the powers of the enemies you kill.
The problem is, I keep attacking the enemies that say '0/1' as they seeeeeem to be the first levels to get through? Like, I keep trying to attack the weakest enemies I can find, but I keep dying super easily and it seems even the weakest enemies is like a hundred times stronger than me. That's why I think I'm missing something or doing something wrong.