Hearthpwn

No matter what I do, I always end up losing to combo priest as highlander mage. 

 Sheep, blizzard. 

and always kill his minions before he buff them.

Flame Ward

You can't beat anything with any mage deck, Blizzard completely destroy the class with endless nerfs and Hof's.

you don't

It's probably one of the worst matchups in the game right now tbh. I think the best the mage can do is try to get off a big doomsayer/freeze turn, but that requires the priest player to overcommit and not have topsy turvy in hand. 

I managed to beat it actually. But I think he just drew bad and I was able to overwhelm him with removal and board clears. 

Easy sheep any deathrattle card like Khartut, keep pressure up early and often, and use Dr. Z to play the priest silence card....bounce him back if possible. Alexstrasza him to 15, have a pyroblast ready to obliterate him. Puzzle box for only desperate situations.