Halo 3 Campaign Complete Legendary achievment : FPS / MMORPG / RTS Discussion

•Make good use of checkpoints. If you need to, run around and die once to get a feel for where the enemies are and what course of action you need to take.
•Be cautious once you know what to expect. Take it one enemy at a time if you can. Make good use of your grenades. If you have plasma or brute grenades, try to get a stick on one of the bigger enemies to take them right away. Some may require two, but it's much faster than picking away with a battle rifle.
•Let your NPCs do some work for you. They're not exactly powerhouses and you can't let them do everything, or they'll just run out and get killed, but they can lay down some initial fire and draw the enemies to them, then you can pop out and get more shots off without taking damage.
•Your shield recharges, make use of it. If you get hit once, more are sure to follow, so find some cover and recharge. The enemies will usually watch the area you went into cover, so if you can sneak around to another side from there, you can probably catch them off guard

Beat the campaign on legendary.