Too easy, and the bonuses from early calls are too high - spend most of my time hovering over the call button. Not sure what gargoyles are for; had consistent wins with just skeletons and ghosts.
To get your hero off one boss (so you can attack another), just move him to the ground first, then click on the 2nd boss.
Funny all the critiques here, there are a lot of badges being given out for 3-star play, so obviously some people are making it work just fine.