Fracture

The Catapult is called the IFR-19. It is very similar to the Black Widow, the Mole Mine and ST-4 Torpedo Launcher. Simply pull "RT" to fire then "X" to detonate when the shell is in the vicinity of your enemy. One difference with this type of ammo is that IT RICOCHETS OFF OF HARD SURFACES and goes a few times before it detonates on its own. Remember this for later.

The first time I remember coming across this weapon is on the "Washington, D.C." Level. After fighting through the many buildings and streets, you are going to come to a dead-end and the Lieutenant will say something like "I think I've found a way to the other side of the...(something, can't remember)". At this point, you have to double-jump or raise the terrain to get into a building, marked by your way-point, and then you have to drop down a hole in the floor. You will get a save checkpoint at this time.

Go around the corner after dropping down and you will run into the IFR-19. Pick it up and stay in the building as the enemies come running out of the alley on the right. Then, let them have it. If you go through the alley they came out of, then down one or 2 more alleys, you can get about 15 or so enemies with this weapon. There is ammo along the way. Stop before you get into the next courtyard area. Then, load the last checkpoint and repeat until the achievement pops.

Also, for some extra physics fun, you can use the ricochet feature I talked about earlier and try to bounce a shell into the alley before they come running out in the open.