Simple buff and not overpowered at all:
Give his Mana Burn damage 20/30/40/50% splash on an AoE.
That is it. That is all he needs to be viable.
It would help him farm earlier as well as punish clumped-together opponents even more than before.
It would even make sense lore-wise, since he would be using the precious mana those filthy mages love against them.
Please. Give him some of that sweet, delicious power creep we all hate to love and love to hate.
Please, make Anti-Mage a good carry again.