This achievement is buggy.
To throw and enemy you must grab them with
and then press
again and aim in the direction you wish to throw, there are many windows in the beginning levels and lots of chances on the level 'Take Out the Trash' to throw them off a cliff.
However if this method doesnt work then there is an easier method near the end of the game, on the level 'Pound for Pound' there are SUVs that drive through the level, any henchman these SUVs hit during the level count for this achievement.
A good place to farm this is after bolt gets the 'Earth Shake Ground Pound' after you defeat the enemies in the warehouse area.
Dash out the door (
) and go invincible to destroy the first SUV, continue running to the corner until the second SUV appears, let this one run you over and kill you
. You restart just after you exited the warehouse so keep repeating until the achievement pops.Note: If the first SUV kills you it doesnt matter as it still hits a few enemies.
Here's a video to show you what i mean.