I did this on the High Times level (mission 1 of Alien World Order), shortly after smoking the hookah (the one related to the "I Never Inhaled" achievement). Upon entering the next area, there's a huge fountain in front of you. Follow the red "detour" signs around it, which will spawn an RPG to the left of where you entered the area from. Just keep killing enemies until the Firefly Troopers (flying aliens) start spawning, wait a few seconds until they're small, then blast them with the RPG, or shoot the buidling behind them, the RPG's splash damage should take them out as well.
Alternatively, of course, you can also enter the cheat code to get the option to unlock all weapons.