Instructions
Cast fire-based attacks and spells to deal damage to Vexen. Avoid frost attacks, since he is immune to them.
Attack with the Omnislash and Ars Arcanum sleights while behind Vexen to deal heavy physical damage to him. These must be from behind, because Vexen's shield reduces physical damage he takes from the front.
Use Parasite Cage to remove the effects of Vexen's auto-life affect, preventing him from regenerating health.
Use dodge rolls to avoid the ice traps and ice needles. Getting trapped in a freeze trap will allow Vexen the time he needs to get close and attack you.
Cast heal cards if your health drops low. Continue dealing damage to Vexen until he goes down.