Whenever you find yourself in a stutter, and there are enemy guys roaming about freely, then they have harnesses you can destroy.
There are basically 3 kinds of enemies wearing harnesses:
1.) Normal enemies who simply move within stutters.
2.) Enemies with larger harnesses that can dash about, and be annoying.
3.) Juggernauts who you can only really take down by destroying their harnesses.
As I played through the game, I found group number 1 to be the easiest to farm. It only takes one bullet from your pistol to destroy their harness, and they are pretty dumb, even on Hard, so sneaking up behind thm is not that difficult.
Group #2 takes a few more bullets to take down, and you can not really stop them with your time powers. They are way to spastic to get consistent clean shots on their gear.
Group #3 are gimme's, but they are few and far between. I am almost done the game, on Hard, and I have only seen ~5 total. The big guys alone won't snag you this achievement. FYI: Using a shotgun or Light Machine Gun cuts them down fast. Using the Time Rush ability, I would simply run up behind them and fire away a couple of times, and they were toast
I found a decent place to grind this out was on the bridge during Act 4. Getting to this place has a few of the #1 enemies, but the sweet spot is after you make your way past the roadblock where the game saves, and you have 1 Juggernaut, 2 of the spastic guys, and 3-4 of the 1-hit wonders. There are also cars around that you can throw your time stop bubble around, fire off a few rounds, and get the Area Effect
achieivement as well.
Questions, corrections and neg vote justifications below.