Like all of the achievements that DO NOT say in matchmaking this can be achieved in a custom game.
Load up a custom game with a friend or second controller, SOLO game does not work as the enemy base does not spawn.
Select halo 2, then anniversary.
Select Assault game mode and any map
grab the bomb from YOUR base (me and a friend thought you had to go to enemy base, idiots)
Run across the map and do not get in any vehicles or shoot as this will drop the bomb.
Stand in the designated area and bam achievement.
(both you and a friend can do it at the same time)
Happy HuntingFor those of you that enjoy a video guide your in luck
All credit for this video goes to SilentRich69
He created this video using this guide so follow along.