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Co-op is also available in the Halo Wars campaign, both via system link and over Xbox Live. However, unlike in other Halo games where co-op is available, it is not possible to play local co-op in Halo Wars and Halo 5: Guardians.
Co-op gameplay involves 2, 3, or 4 people playing through the campaign as a team. If one player dies, they can come back to life, or "respawn". The player who dies has to wait until the player(s) who is/are still alive is/are not engaged in combat and is/are at a safe location before they are able to respawn. If all players are killed, they will restart at the last saved checkpoint. Additionally, in Halo 2, if two players are playing on Legendary difficulty and either of them die, then both of them have to start at the last checkpoint. In any of the other Halo games, many players will attempt Legendary on co-op, because Legendary is often too difficult to attempt alone without lots of practice. When a player respawns in Halo: Combat Evolved, they are always equipped with an Assault Rifle and a Plasma Pistol. In the sequels, each level has a designated pair of weapons that respawning characters start with.
In Halo: Combat Evolved, Halo 2, and Halo 4 co-op, when playing a level involving John-117, both players will control two identical copies of John; when playing a Halo 2 level involving Thel 'Vadamee, both players will control identical Arbiters. Halo 2 was planned for online co-op mode for up to two players on Xbox Live, but due to time constraints Bungie was not able to perfect online co-op for Halo 2.
In Halo 3, when there are two players, player one plays as John-117 and player two controls Thel 'Vadam. Certain cutscenes and lines of dialogue are altered to reflect the fact that, unlike in the single player mode, the Arbiter never leaves John's side. In addition, all of the Arbiter's lines of dialogue outside of cutscenes are removed. In three or four player co-op, player three controls N'tho 'Sraom and player four becomes Usze 'Taham, though these characters do not appear in cutscenes, and are not referred to in dialogue. Player one, as John-117, begins each level and respawns after dying with human weaponry such as the Assault Rifle and Pistol, while the other players, as Elites, begin levels and respawn with Covenant weaponry such as Covenant Carbines and Plasma Rifles. Two player co-op can be played in split-screen mode in all three primary Halo games, but three or four player co-op in Halo 3 is only available over Xbox Live or system link, with at least two Xboxes with two players on each.
Co-op is also available in Halo 3: ODST; in the Mombasa Streets sections of the campaign, when player one controls the Rookie, player two controls an identical ODST. During flashback missions, player one controls the main character of the flashback, but player two still controls a generic ODST, even if the flashback character is accompanied by other members of the squad. For example, in NMPD HQ, the flashback is from the perspective of Romeo, who is constantly accompanied by Buck. In co-op mode, player one controls Romeo, but player two controls a nameless ODST, leaving Buck as an NPC. The same goes for players three and four. Just as in Halo 3, two players can play on one Xbox with split-screen, but up to four players can play at once via Xbox Live or system link.
In gameplay terms, each player can access the map and drop their own waypoint, which will show up on everyone's map, but only on the compass of the player who placed it. In addition, all players download all Audio Logs, no matter who actually accesses the terminals. Only one log can play at a time, and any player can stop the playback or start the playback of a different log.
Finally, ODST added a new co-op multiplayer gametype called Firefight, in which players are allowed to pick their character, choose their personal emblem, and decide whether or not their character is wearing their helmet (with the exceptions of the Rookie, who must wear his helmet, and of Johnson, who cannot wear one). For more details on Firefight, see its main article.
In Halo: Reach, the armor that you choose for your multiplayer games is also the armor you will wear in the campaign. This game also brings back the Firefight mode from the previous game, Halo 3: ODST. In Reach, you can still only play two-player split-screen co-op, and four-player co-op via Xbox Live or system link.
Co-op gameplay is very different from single-player gameplay; there are several new strategies and alterations present. Starting and respawn weapons tend to vary between single-player and co-op, as do the starting weapons of each character and the strategies that can be used in co-op—some of which are documented below.
The "coward method" is basically having one player hide in a safe spot while the other player charges forward and kills everything with impunity, charging again and again as they die until no opposition remains, very much like cannon fodder. It is commonly used to beat the campaigns on Legendary. It cannot be used in Halo 2 on Legendary, however, because if one player dies, both must restart from a checkpoint.
This method can also be used by one person playing as two characters on co-op, provided that they can place the "coward" in a safe, unreachable place.
This method involves all players charging at the enemy. This method is regarded to be harder than others. To use this technique successfully, players will need to coordinate movements with each other to avoid wounding or killing each other with splash damage or friendly fire.
Run and Gun
This method consists of attempting to get through the level as quickly as possible, while also trying to get the most points possible. It often involves using 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 service record; they may also wish to get the meta-game achievements by relying mainly on the time bonus for the level. Also these can be done as a challenge for the players or to speed through the story as quick as possible.
This method makes up for the 2 or 3 weapon limit and has players focus on specific aspects of combat, such as enemy types or positioning. One of the most ideal strategies utilizes one of the players using weapons more effective against shielding while another player uses weapons more effective against non-shielded enemies. This way, shielded enemies would lose their shielding rather quickly and become vulnerable to the other player who would have the proper weapon equipped to finish off the enemy, effectively removing the need to constantly switch weapons. Ideally, the player assigned to shielded enemies would utilize Plasma Pistols for their overcharged shot while the player assigned to take out the non-shielded enemy would utilize headshot weapons, namely a Battle rifle or Covenant Carbine, forming a two-person Noob Combo.
Plan is fairly simple: one player attack the enemy from the front, whilst the other player may flank the enemy from behind or attack them from the side. Can be used against the more powerful Covenant (Brute Chieftains, Zealots, Ultra Elites, Mgalekgolo etc.) in which two or more players could gang up to take a powerful enemy down or, can be used to eliminate large groups of Covenant using combined Assault or Battle Rifle fire at close to medium ranges. However the problem is the "flanker" being able to get to the position they need to be in order to flank.
In some occasions, two players may not be needed for this method. It can be done using the UNSC Marines as a distraction for the Covenant whilst the lone player attacks from the side (or flanks). One example of this strategy actually being put into practice is in the level "Tayari Plaza" in Halo 3: ODST, Chips Dubbo suggests to Buck "flank the Covenant on the streets" whilst Dubbo and his team-mate assault the Brutes head on.
Gradual fragmentation is the method of going after one group of enemies at a time, sniping enemies one by one or grinding down all the enemies gradually (hence the name "Gradual Fragmentation"). Requires patience, and good judgement. Can be done on single-player but much more efficiently done on co-operative play, in which this strategic combined with Two-Way attack can be very effective indeed.