If I may participate, the achievement unlocked painlessly for me, after acquiring the magnetic something gun. I just launched an easy enemy in the air and then wipe him (or it?) with a timed (I hold the left bumber for a sec) grenade.
It's the place where you set the prisoners free, and you enter the giant hangar from the roof. Piece of cake!
Try this one first unless you want to mess with the Knights.