Try going by his mana pool. Essentially he never has flame strike until turn 7 and then assume he has both. Makes ur decision making a little easier just assume the worst and prepare to deal with it.
Not easy tho
They have Blizzard and Flame Strike. You can't play around 4 problem cards without crippling yourself.
On top of that Cone of Cold and some run Arcane Expoasion and Arcane Missles as well, on top of the single target removals Frost Bolt, Sheep and Fireball. Frost Nova as well (altough crappy card)
It's a waste of effort to try to play smart. It leads to nothing. You either get lucky and they draw bad, or you'll lose board control and/or be nuked to death. There is no secret "strategy" involved fighting mages. Really, there isn't. If they would ONLY have Flame Strike as board clear, it would be easy to play around that. 7 Mana, 4 Damage. Play accordingly. DONE.
But turn 5 and/or 6 2 dmg + freeze for 5 mana, and prior to that AE and Cone of Cold (which lets you lose almost all trades the round afterwards + the stall for another round, so they get access to another clear spell) can't be accounted for. They have possible 4(!) turns in succession where they can destroy your entire board, and prior to that trade nornally or nuke single target or stall / aoe your minions down.
Blizzard should only deal 1 damage and Fireball should only be castable on minions. Those two fixes would make the class so much more enjoyable to play against.