"Six and a half"

The answer is very simple: 1) Necromancy is used to obtain 1 skeleton (so that the same hero can exchange the djinn for a crystal, + necromancy distracts by gathering skeletons). 2) Necropolis - because of the skeleton converter. 3) Behemoth - because of its low speed (a bone dragon could also work - I just didn't bother). 4) Demon - just a picture. I didn't really aim for a storyline or logical scenario, sorry... P.S. There's no need to send anything about Fiura - I'm just being slow.