Thanks both of you guys.
The first one is a monster that you cannot bring down below 1 hp unless you use a certian skill (Let's just say for the sake of discussion, Seal). So any other attack/skill or anything else cannot bring the creature below 1 hp. You can bring the creature down to 1 using everything else, but at 1, only Seal can finish it off.
EDIT: I typed in Blizzard's little tricks but it didn't show anything.