High OTK potential with arcane bolts and/or frozen touches. Bonelord frostwhisper lets you increase the damage dramatically if you are playing against a deck with abnormally high health like druid or blood DK.

Best case scenario you can generate six arcane bolts (not including commander sivarra) with your minions but that won't happen every match so the following example combos will be with different amounts of bolts. And the damage can always be supplemented with frozen touch.

(5) Vexallus + (2) Magister's Apprentice + (0) Arcane Bolt = 7 mana deal 24 dmg with 4 bolts or 36 with 6 bolts. 

(4) Guild Trader + (2) Magister's Apprentice + (0) Arcane Bolt = 6 mana deal 20 dmg with 4 bolts or 30 with 6 bolts. With spell damage any frozen touches added would increase damage by 5.

Bonelord effect + (0) Vexallus + (4) Guild Trader + (2) Magister's Apprentice + (0) Arcane Bolt = 6 mana deal 40 dmg with 4 bolts or 60 with 6 bolts. With spell damage any frozen touches added would increase damage by 5.

There are more high damage combos within the deck especially if you have the Bonelord effect active. With chip damage from minions and your hero power in most games you won't need your max damage combo to win. With the high variability of your possible OTKs it would be impossible for this deck to be disrupted by one off cards like Theotar.