This is the fastest way I know. You will need two controllers.
Load up Bloodline in Halo 2: Anniversary Forge. In the options, turn health down to 10%, shields off, and weapon damage all the way up. Make sure respawns are set to instant as well.
Once game starts, pick up any respawn point, click B and choose 'Delete All Of These'. You may have to do this to several spawns before they are all deleted. Place a new respawn point anywhere you like and place a gungoose beside it. Have the second controller enter player mode and kill itself so it respawns at the respawn point. Get on the gungoose with your main account and open fire on the second account. It will respawn instantly. Use a rubber band to hold trigger down and place crosshairs on spawn point. The second account will respawn in the same place every time if this is the only spawn point. You will get approximately a kill every second this way and you can simply walk away and let the game play itself.
If your controllers are wireless, they will die after a short time so get creative and attach them with rubber bands to a rotating fan so that each time the fan turns, it moves a thumbstick. This will keep the controllers from dying as simply keeping a button constantly pressed does not work. You will need to use forge objects to block the gungoose and keep it from moving.