I had him as 10th boss and my paladin deck was really op and had not problems at all before but i stand no chance against this boss. If you cant clear the first minions the turn he plays them he just gonna wipe your board and face with his spells all the time. And clearing them is kinda hard because he has a mana advantage on you.
Completed with warlock, got the spell where my minions above 5 mana, cost 5 mana and a lot of big demons like 2 Malganis and 4 Jumbo Imps. Then i had also 2 Bloodriver Guldans so i just spammed demons from the beginning. And also... ALWAYS i traded his minions!
I also had a really OP buff priest deck, Vargoth as 9th or 10th boss and I lost so easily to him. In 6 or 7 turns. Gotta have some kind of removal spells or big minions fast otherwise it seems undoable. And a good dose of luck ofcourse. He was barraging me with 8dmg greater arcane missiles.