The easiest way is to board several gunships with 2-3 enemies on it. Simply attach to the gunship's hook, dismount and use undertow to push all the enemies off. KEEP AN EYE PUT FOR SALTS!!
If you keep running out of enemies to push off keep loading up chapter 'Hand of the prophet' You don't need to hit a checkpoint to save your progress, simply keep reloading until the achievement pops.
GOOD LUCK :)