This can be done in Forge with two controllers.
Load up Bloodline in Halo 2: Anniversary Forge. Pick up any respawn point, click
and choose 'Delete All Of These'. Place a new respawn point anywhere you like and place a gungoose beside it. Have the second controller enter human-mode and kill itself. It will respawn at the respawn point. Get on the gungoose with your main account and open fire on the second account. It will respawn instantly. Kill the second account 24 more times. Easy as pie.