Has anyone beaten the boss?

[Flippy]

I did it with the fire guy, maxed brutality is really useful and fire is great during the run, even if useless against the boss

[Bread4Ducks]

Managed it with Flayed, having lots of shield is a massive help against the boss. Giving Serenade a lot of the attack cards you find helps as well because their starting deck is awful. If possible you want to play a discard in the previous fight that lets Serenade use their freeze because it means the boss won't draw a new card in the first round.

[BarryChopstickles]

Also beat twice with Flayed back-to-back. Very easy with shield stacking, can negate 3-4 DMG every turn

[Jai le Leu]

I beat him the first time I encounter him with Poisoned and Serenade, and a Wolf trap equiped. I didn't pay too much attention as i thought I would die quickly, but it ended I won. I had a lot of gold teeth to throw at him

[Ephesos]

The key I found was to give myself all the cards with loot (and load Serenade with the rest to not dilute my deck) and to freeze the boss with Ice Trap repeatedly. Get rid of all the flames and freeze the boss to stop him from generating more of them.

[Gabzou]

Global tips:
-Get items with high damage
-Prepare your hand before reaching the boss: you absolutely need one loot to remove fire, two is better, otherwise some cards will destroy you (3 dmg or 3 shields for the boss will make beating him almost impossible).
-Getting max mutations makes it much easier (especially the 2nd item imo)
-Try to leave a green card in your discard for the freeze. Freeze is overall excellent especially for the angry boss.
-Try to reach the boss at ful health. The sword transformation heals it, but change it's max HP from 4 to 3. If you have lots of heal, you may want to keep it not transformed.

Poison guy:
-Max brutality for that poison at turn 0 which is 3 free dmg.
-keep your gold teeth for extra damage as much as possible

Fire guy:
-Max brutality again for that free 2 damage.

Shield guy:
-Abuse your auto shields, you may need a damage item.

Last guy:
-Try to max purple and abuse that +2 shields when using the "gear" symbol.
-Incredibly easy if you manage to max purple and red.

Sword:
-One of its cards is totally useless since it does not reach the boss. Give it more useful card to reduce the probabity to draw that one.