Before the last tower you have to climb, near the end of the game, the level before you fight the Giant Stroyent Creature who flies and shoots rockets at you is the level where you receive this award. The award consists of saving the Marine, Tech, and Medic who accompany you. There are very few enemies and it's really easily done. As long as you lead, they can't die. Just be careful of the Gladiator in the room right before you take the teleporter. After you beat the level, the award is yours. Incase the level description wasn't enough, it's the level where at the start you hear the voice in your headset saying a marine blew himself up with a grenade. Its very easy, even on General to save these guys.