Too OP. Make him a pure buff hero by drastically reduce the damage to enemies. E.g., damage to enemy = 0.01%*magic attack, which is a nominal amount.

Otherwise, make his buff/damage randomly choose either enemy or team. Also, can consider add in debuffs too into the mix (i.e. slow: lose 1 attack turn, armor-break, etc.) Then here crit chance will play an important role.