Cooperative play
From Halopedia, the Halo wiki
Co-op is a multiplayer gametype found in all Halo games except Halo PC. It allows two or more people to play the campaign together. If one character dies, he or she will respawn with the weapons they begin the level with. The player who dies has to wait until the player who is still alive is not engaged in combat and is at a safe location to be able to respawn. If both players are killed, both you restart at the last checkpoint. Additionally, in Halo 2, if you are playing on Legendary and either of you die, both of you have to start at the last checkpoint. Though in any of the other Halo games, nearly all players will do Legendary on co-op, because Legendary is often too difficult to attempt solo.
In Halo 3, when there are two players, one player is the Master Chief and the other is the Arbiter. It is somewhat the same with three players, with the addition of N’tho ‘Sraom as another playable character. If a fourth player is available, the Elite Usze ‘Taham will be playable. It should be noted that the players can not choose who they play as and the game will randomly select who plays as who. Co-op can be played on split screen on Halo and Halo 2, but can be played either split screen or over Xbox Live in Halo 3.
Co-op Game-play
Co-op game play is very different from single player, there are several things that you can do (with the exception of Halo 2) in regards to game play, many people don't realize how different co-op play is from single play, for instance, when you die in co-op play in Halo 3 you respawn with two grenades/plasma grenades (depending on if you are an elite, or if you are the Master Chief), a full Battle Rifle/Carbine and Assault Rifle/Plasma Rifle. In Single play, however, you start off most often with different weapons, such as a pistol and assault rifle. In single play the Arbiter has a range of devices and weapons (anything from the energy sword to an auto stealth unit in his armor). Conserving ammunition is a big part of single player as well, as running out of ammo is a common problem. Brute Chieftain's are generally extremely difficult to kill, as they are surrounded by brutes, and any one brute can kill you, and they carry weapons of enormous fire power, often only needing one shot to kill you.
Strategies
Coward Method
First and foremost, the "coward" method is basically having player A staying/hiding in a safe spot while player B charges forward and kills everything with impunity. This method is frowned upon greatly, and is used commonly, though it cannot be used in Halo 2 on Legendary. The best way to use this method is to play as the Arbiter in Halo 3, because he has plasma grenades to stick enemies with, and to alternate players so that everyone gets a turn, or you can have one sniping, so the player is not necessarily safe, and is contributing.
The variation to this method is single player, where it is actually one person playing on co-op, using the method stated above.
Full Shotgun
Both players charge the enemy, though this method is harder, it will make a more enjoyable playing experience, and will be a true test of skill to get through levels, though this is useless for single player experience. To use this successfully, you will both need to coordinate movements with each other so that splash damage attacks do not hurt the other player
Run and Gun
Attempting to get through the level as quickly as possible while also trying to get the most points possible. This method often involves vehicles and ignoring many enemies that you may otherwise be inclined to shoot. The reason players do this is generally because they want the Legendary symbol for their Xbox Live Account, or they may wish to get the meta-game achievement by relying mainly on the time bonus for the level.
