I did this in Act 2, the 13th mission, in which you infiltrate a vicinity to rescue someone.
After completing the first objective that is to eliminate all enemies, you'll hit a scene where reinforcements are called in. Once you regain control, you'd be prompted to press X to neutralize these new enemies as they drive above a steam pipe. Wait until the cars are just reaching the steam pipe, and activate focus. Wait a bit more until half the car has driven past the steam pipe, and press X to neutralize them.
If done correctly, the explosion will kill four enemies. The key here is the timing of pressing X.
If you fail in the attempt, reloading the last autosave will put you at an even better vantage point to see the cars and the steam pipe clearly from higher ground. Just work on the timing of when to press X, and you should get the achievement in no time. Good luck!