Mulligan for Hungry Crab AND Arcane Blast (for his turn 1 Tidecaller). Play coldlight when he is at 10 cards.
Tons of methods to get 1/2 aoe. With doomsayer + Freeze, 1 flamestrike if he plays Warleaders/Seers.
Bran is for the Bran + coldlight combo to mill him from his unfair 40+ card deck late game, and to refill our own hand with more AoE!
If you don't have Flame Leviathan, just sub in another flamestrike. Levi is just a nice free AoE board clear, plus 7/7 minion swing to challenge the warleader next turn.
-One of the commenters mentioned Counterspell. An EXCELLENT idea! If you don't have some of the epics, definitely sub in a counterspell or two!