Using your Intel View, you can see enemies through walls in areas that you have wiretapped (or anywhere if you have Vito upgraded enough). Just look for an enemy that is right on the other side of a locked door - there will be plenty of opportunities. Press Circle to kick down the door instead of pressing Square to crack it open, then immediately go into slow motion, aiming and headshot the enemy on the other side.
An easy place to get this early in the game is during the prostitution racket in Delray Hollow. When you are freeing the strung-out girls upstairs, the door on the far left is locked and has an enemy inside.