I had to reinstall to get this one to pop along with Frustration Leads To Anger. You basically have to experience all the attacks then die.
List of attacks:
-Summon poisonous plants
-Flaming sphere projectiles
-Small green/white homing projectiles
-Large slow moving sphere projectile(you can parry it and reflect it back if you want! )
-Bullet hell green poison projectiles
-Hex spell (poison gas fills most of the arena around 50% health)
-Curse spell (@ around 75% health summons 4 mini versions of himself which need to be killed).
After you've checked all that off just die from the boss!