TimeShift

The way I went about getting this achievement was as follows:
1. Be sure to check dead bodies for Clutch Grenades by running over them
2. When you have one or more and come moderately close to some enemies stop time (LB + Y)
3. Walk right up to an enemy and throw the grenade at them (RB) This grenade will stick to them.
4. Back away so that you are not caught in the blast. When time starts again the grenade will explode and the enemy will die.
5. Repeat. Do this 15 times total and the achievement is yours.
Doing this I was able to get this achievement early on in the second act.
Alternatively it may be possible to just save at one point, such as in "New Blood" where you emerge from the tunnels to a group of enemies, where you have a Clutch Grenade, run out, stop time, stick the enemy, kill them, reload save, repeat. I have not tested this so confirmation would be great but I don't think it would reset your count if you didn't save, though it is possible.

On the 2nd level in the game Back Again (The first real level where you have access to your time powers) you will see an explosion and walk through a cement pipe with some friendly soldiers on the other side fighting enemies behind a fence. It makes an auto save in the tunnel. One of the friendly soldiers will die and drop 2 Clutch Grenades and once one of the enemies either jumps down from the wooden planks on the far wall or is standing up top shooting walk out of the pipe stop time and stick him with a grenade. If you turn to your right there should be another enemy soldier already on the ground that jumped down and you can stick him as well. After they both die you can reload the checkpoint and keep getting those 2 kills over and over until the achievement unlocks.