Since there are no trophy guides anywhere for this DLC (that I can find), I'll put this here:
You'll have to fight an evil version of yourself who will have all weapon attacks from different types (sword/shield, big sword, etc.). I recommend using a stick-and-move strategy while mixing in charge attacks that give invincibility frames.
I also found that if you stand in the upper-rightish part of the screen (where the ground looks like rubble), that will make Evil Will sometimes get stuck where he is. If you want to do this, use one of the DLC familiars since they have good damage while you stay in one spot away from Evil Will (I prefer Gas Suit).