A great spot for this is at the end of Episode 2, on the level entitled "Overlord". Once you have access to the boss chambers, you'll find a circle of alien eggs which can be destroyed with a well placed shot at the nearby explosives. Save, wait for them to hatch and try to take out a group of them with your RPG. If you fail, you can always reload. This is also an excellent spot to try for the Rage Engage trophy.