Load the Echo map "Crash Site", and get to the part after you beat the 2nd boss. After this you will enter a room with a dropkit, making sure to reload your thumpers if you're done a few. Next leave this room, and sprint to the end of the walkway. You will notice some Burnout enemies, and a chopper coming down the hill (above) in your direction. Thump the burnouts into the air at the end of the walkway, and the chopper should run through them, killing at least one. If not, reload and try again.
Video Walkthrough: