The achievement requires you do three Cross-game playlists
(These include missions from Halo 1,2,3&4)
Quickest playlists to complete under the cross-game section for this achievement are
1. Get in the ring (3 missions)
2. Guilty pleasure (3 missions)
3. The Master Chief Saga
(This playlist is 45 missions long as it has every single mission from all the games but it is required to unlock another achievement for its completion so you'll have to do it somewhere along the line if your planning to 4500G the game)
The Achievement unlocked for me later on in the day several hours after I did my final third playlist, so if you don't get your achievement for doing the requirement it will unlock eventually.