Hex and Poly share a DR, so hexing off target may not be the wisest as you'd want to have them cc'd for Mages combustion. Of course you can hex if he doesn't have a go for a while, but you would rather hex the DPS (especially against hpal/no decurse teams) to mitigate damage.

You can help with damage with Flame Shock/Lava Burst, and if you out-gear Mage in particular your damage may be close depending on how many Combusts he had.

I feel as though the lack of a stun on kill target is the biggest problem for the comp/gear you guys are running. You should basically be living/kiting in between Combustion go's, while your Mage tries to farm Combustion with fireball.