This can be done while using the M79. After leaving the wreck site, you have to go through a platform section and when you're finished with that kill the enemies on your side of the rock formation as well as the guy standing on top of the rock formation. He shoots at you with a M79. Climb on top of the rock formation, pick up the M79 and wait for the enemies. Make sure that you hit three of them at same time.
Credit to: KittyGotWet for the video