Monsters have skulls?
I should probably know this already. I would guess that these monsters are Elite, or champion. But, what are the skulls above the monsters picture on the pre-battle page?
That shows what level champions they are 1 skull up to 3 with 3 being the hardest. Most have increased hit points, empower and pritty much never miss when they attack.
Also they are the only type of monster that drops excellent, mint and legendary equipment.