Having trouble killing this dude? here are some ways

1: Smite
After killing enough enemies you should use smite only on the boss. if you hit him with smite about three times it should kill him or at least get him very low. This is the easiest way.

2: Projectiles
theres 2 weapons that shoot these harpoon things. after using these and some other weapons you should get the boss low enough to kill him using berserk.

3: Katanas
This works well with number 2. there are about nine Katanas in the map (including the one at the start). When the boss performs a melee attack (like swinging his staff or a kick or a punch)
After this, this should give you a chance to hit him three times using a katana. After hitting him three times move back and wait for him to perform another melee then repeat.
After a while it should be safe to use berserk and kill him.

I hope this helps!