I haven't played Halo 2, but the original Halo on PC is an excellent gaming experience on legendary. Except for the notoriously boring Library levels it's some of the most interesting solo play I've ever gotten out of an FPS. It is still my default time filler game when I've got just a few minutes somewhere. The key to beating legendary is aggression IMHO, just attack, attack, attack. If you spawn a group of the Covvie AI and give them time they'll take pretty decent positions around you and pin you down, so take the fight to them and stick a grenade down their throats.

If you don't enjoy Halo on leg. you probably don't like solo FPS games in general.