The map loop has an upside and a downside (Though you're always upright from your perspective) and in the middle of each side is a small grey tunnel that takes you through to the other side.
If you jump in an leave the controls the player will keep flying back and forth through it, then have the person who is going for the achievement aim into the tunnel to kill the player in the hole. I found a good weapon to use is the Grenade Launcher as there is some room for error.
Setup a match with the following settings:
• Map - Loop
• Gamemode - My First Deathmatch
• All other settings default.
Here's a video using 2 controllers