frost nova, cone of cold, frostbolt, blizzard, archmage flamestrike.
then chuck in some spell power creatures or just weenies to deal with early game.
You can never attack because your board is always frozen and the turn you dont get frozen you get pyroblasted and the following turn it happens again.... game over.
No creature you play can kill them unless you get extremely lucky so the only way to deal with this deck is mass healing/ decking them or having the same deck but getting the dmg earlier.
tldr mages are OP, the only counter is mass healing decks or other mage decks.