Get a second controller and go to Forge on the Halo 2 Anniversary Map "Nebula" and kill the guest character and let him re-spawn in the middle of the platform. Since the re-spawn time is set to 0 you should be able to go up behind the guest account, punch him in the back and step back
. This ensures that the guest account re-spawns quickly and on the spawn point instead of falling into oblivion.
If you got a few in normal multiplayer then it will only be necessary to get the number of kills required to top it off.
You can also use this method to gain the "Double Down" Achievement as well.