well, i netice that with vexens attacks, there is a very high chance to evade them, so keep on the move, and if it seems as though it is impossible to evade the attacks, then use zero cards. now the only way to hurt him is to attack him from behind, so if you are in front of him, dodge roll behind him, and hit him with a good combo. I DO NOT SUPPORT PREMIUM CARDS FOR THIS BATTLE, espically if you are having trouble with him, since they cannot be reloaded, and you are having trouble, its gonna be a long battle, and using premium cards, will lead to a very short aount of cards. hope that helps!