The hardest is probably the recon armor and this is because you have to be or know people in bungie. The other way is for something weird to happen such as the traffic cone kill.(look it up on google...
To earn the Overkill Achievement in Halo 3, you must win the Overkill medal (kill four players within four seconds of each other) in a ranked free-for-all matchmaking playlist such as Lone Wolves.
if your talking about which is better i think halo 3 because there is no multiplayer in halo 3 odst only firefight wich gets boring halo 3 ODST is mainly solo fighting (lone wolf stuff) but halo 3...