One of the easiest ways to do this is in the final mission of Episode 2 "Overlord". In it, there is an area with 20 unhatched alien eggs and a bunch of explosives right before the final boss. Normally, you could shoot the explosives to take care of the eggs but instead, save your game and let them hatch. Let them cluster up, activate your steroids and fire away. You should easily be able to get this trophy. If not, you can reload your save and try again.
This is also an excellent spot to try for the following trophy.