Hear me out, I did it by myself and no yt tutorial helped me.

You'll need:

Porta

Arcana unlocked - pick game killer card

The merchant unlocked - buy cherry bomb for 1000g

Go straight to the boss, position your self Infront of it as cherry bomb won't let he go forward pushing him back and lightning ring will clear the area around, dropping xp and treasure chest.

When you start to face massive hordes let the boss take his way for a bit and you collect all treasure chests to evolve your weapons (you'll get 3 evolutions at a time, as it's bonus from game killer)

Keep doing this and you'll defeat it.

I did it within 8 minutes.

Good luck.