Every Boss has 2 unique skill, some can penetrate your defense, some can drain your life and turn to theirs, and the most powerful skill I think, is double strike. Allow me to explain below:
Double Strike: Attack twice in one turn, it is dangerous when Boss in high level. (every time you defeat it, it grows up one LV until you die) If you don't have enough HP. It's possible to kill you in one turn, or worst, force you drink 2 potion and still die. So I won't put my bet on boss who can double strike, it just speed your death.