I got 16 of the 21 stunts without realizing it, but still had 5 to go. I spent about 15 minutes trying to melee an airborne enemy to no avail. And since at the point I knew I wasn't gonna go for the platinum, I figured I wouldn't waste any more time on the stunts either. So there are 4 other stunts I don't even know what they are. So some of them were a pain in the ass huh?
At least I seem to remember that. The ones that gave me the most trouble seemed to be on the evil playthrough. I remember going to youtube to find a way for a couple of them. I want to say it was attach a sticky bomb to an enemy that is in the air. So you have to do the repulsion blast thing and then switch to the bomb and get it on them before they landed.