Well, you don't need 3 people, because I got this with 2 controllers, and just me playing both. I got Double Down, Decorated Warrior, and The Goose is Loose using this method below:
Halo 2 > Forge > Bloodline > Options > Shields & Health > Damage Resistance = 10%, Shield Multiplier = No Shield >
(back) > Weapons and Damage > Damage Modifier = 500% >
Now, remove all spawn points (brag object (spawn point) with
, Scroll down to "Delete All of These". Might have to do this a few times till all are removed from map.
Place new spawn point (
Place Object > Spawning > Respawn Point). Do again, but go to Vehicles > Mongooses > Gungoose
Have player going for achievement hop onto the Gungoose, and sit in front of what should be the only respawn point.
Move 2nd player over, kill them with 1st player, and they will continue to spawn nearly instantly in the same spot.
I did not intend to go for this achievement, as I was working on the 25 Gungoose kills.