Mr. Gency said:
Shoot their limbs!
Anyway, in Splinter Cell, mark everyone, even if you don't want to execute. If you do, you can see them through anything. Also, in co-op, you can execute your partner's marks and vice-verse and you can join your partner's execute if the game goes into slow-mo. Not only that but one melee kill earns both players a execute. Also, if a marked enemy is in melee range an execute will take them out with a melee kill, earning you a second execute (I haven't tested it with "Death from above" or pulling an enemy from a ledge, yet). And finally, during the part when you have unlimited executes (the game is even kind enough to mark enemies for you)this is you opportunity to get the "15 kills with a rifle without reloading" or the "get 10 kills with a pistol without reloading.