TimeShift

Pick a time when you've got 3 enemies all very close to one another and a full time power bar. equip the shotgun, run up to them and pause time. Fire one shot into each enemies head, then keep shooting each in turn until time unpauses - achievement unlocked.

I found this was easy to do on act 3.

As you drive a quad bike through a tunnel you come a pool of water with electricity. You have to stop time and jump it on the quad. When you land you get a loading screen.

Once the game has loaded you have a very short way to drive to the end of the tunnel. As you get near to the end of the tunnel you will see 3 enemies walking away from you.

Get off the quad, get your shotgun out and sneak up as close as you can until they say "weapons ready" or "weapons hot". As soon as they do freeze time and put a few shots in each. They should all die and the achievement should pop.

If it doesn't you are right at a check point so just let the survivor kill you and do a quick load and you will be right there with the 3 enemies ready to try again.

Hope this helps.

While playing through the stage "New Blood" you'll wander through a sewer/maintenance tunnel. You'll have to hold X to turn a wheel on the wall which in turn raises a barrier. You have to then activate your time powers to get under the barrier before it lowers again.

Immediately after this you'll be climbing out of the tunnel into what looks like a park. This park has a number of enemies in it.

All I did was climb out of the tunnel, kill the first enemy I saw to get their attention and then drop back into the tunnel. Use your radar and wait until you see at least 3 of them clustered nicely at the top of the ladder. Climb up, Stop Time and shotgun 3 or more of them. I only had to restart once while unlocking this achievement.

You can easily get this on the 2nd level Back Again (The first real level where you have access to your time powers) on the first three enemies. When you come out from cover walk towards the first enemy and stop time and kill him with the pistol then turn to look at the other 2 enemies and use the LT secondary fire of the assault rifle you picked up from the first enemy you killed. You should have enough time to kill all three as the grenade secondary fire should kill both of the other enemies with one shot. I killed all three during one time stop but not all at the same time that may be because they didn't all die until the time stop wore off since time is frozen.