FYI the bosses have scaling hero powers... if you get him early he gets a trogg each time you cast a spell. If you get him later on, he draws a card and changes it's cost to 1 each time you cast a spell...
I had a run ended on 7 by that evolve boss, his turn one was living roots + evolve (1 mana, +3 cost... he starts with 2 mana by then) into a 4/5. It got worse from there.