After running away from the Helicopter in the Aquaduct (on foot) and heading into a run-down dungeon-esque building, there is a slippery slope leading down and right with 2 explosive barrels at the top. Move one of the barrels to the top of the slope, and create a save file. Push it down and it should roll and get stuck on barnacle "tongues" and be lifted into the air. The barnacles are behind a ledge, so you must shoot the barrel until it is flaming. Time it so that the barrel explodes as high up as possible. Reload the save if you do not kill all the barnacles at once.