Mr. Shifty

First off, you need a weapon to unlock this achievement. No enemies die in one punch without one, and one slowmo isn't enough time to throw 12 punches.

The best place I found to get this is Stage 4 "Defiant". Progress through the level until Nyx teaches you about slowmo. Then use the first group of enemies to build up your meter (this should be fairly easy, though there are plenty of sticks around if you're struggling). If you kill them all and your meter isn't full, allow the next enemy to kill you so you can restart the room.

With a full meter, kill the next 2 stray enemies, being careful not to let a bullet get close enough to trigger slowmo. Again, if it does, let someone in the next group kill you. Once these 2 enemies are dead, break one of the many ladders around the room to get a stick. If you proceed downwards through the room you'll find a group of 6 enemies all clumped together, with another one or two nearby. Enter this room, trigger the slowmo (careful not to die!), then go crazy on the enemies with your stick. This should net the achievement, but if not, there are more enemies in the room so allow one of them to kill you and try again.

Hope this helps!