Mountain King seems to be pretty underwhelming when it comes to dungeon bosses. I mean the only bosses where he could be able to stack his passive are spider, Minotaurus and ice golem. And ice golem and Mino are the only really efficient way to stack his passive. Why does it need to reset each round? Just let it go through the whole battle to actually give him the title of "King of the Mountain". Don't be rude. Let him destroy Smaug, give him his Arkenston back and let him become the "King under the Mountain" . 

Greetings