You'll pick up your first Project X weapon in Level 4 - Vengeance, can't miss it. Shortly after picking it up, you'll detonate a door which will disable the power in this room allowing you progress on through. Go in the room you just blew up and pick up the credits and the Flamethrower inside and when you head on out, a bunch of aliens will group up at the entrance waiting for you to come out. Throw one or two Stun Grenades at the group, switch to the Project X and let it rip! You should easily be able to kill 5 of them in one hit.
If you're having trouble earning it here, not to fret! The rest of this level pits you against a ridiculous amount of enemies, more often enough with a bunch of Larval enemies. Use a Stun Grenade if you must and fire a shot with the Project X.