Barnacles are the hanging enemies with the long tongues. Use one explosive barrel to kill five of them.
The best time to get this is immediately after you first run away from the helicopter in the aquaduct. You'll go through some tunnels and find two explosive barrels above a slippery slope. SAVE your game now. Drop one of the barrels down the middle of the slope, so it will be picked up by the middle barnacle at the end. As the barnacle pulls the barrel up, shoot the barrel a few times until it is flaming. You want the barrel to explode as high up as possible to kill all five barnacles there. Reload if it doesn't.