Welp, got stuck at the 4th boss too. The controls were a bit confusing but I’m happy it was possible to change them. Great bosses, I love the art style and it was impressive you managed to make so many and even randomize them !
Ah, that means you need to upgrade. 4th boss is the starting stone for upgrading yourself.
If you're having problems completing Survivor mode, you can try Hunter mode which allows infinite replays to upgrade yourself to defeat the bosses :)
Thanks for playing :)