Co-op is a game mode in the Team Fortress series. In this mode, the player (and some friends, if he/she want them) must beat the single-player levels in the original Quake. Classes can be picked when the player returns to the Introduction map via beating an episode.
In Co-op, ammo and Armor (but not Medkits) automatically respawn after a certain amount of time. In addition, all grenades held will disappear when a saved game is loaded, even if the loaded game is in the same map that the save was loaded from.
At the start of a map, the player will spawn with roughly half of the maximum of everything except health.
Difficulty: Easy to Hard
Depending on one's play style, the Scout can be the easiest or hardest class in Co-op. If you wish to simply get to the exit as soon as possible, then the Scout is the best choice. His speed plus Concjumping will allow you to blaze through maps in no time.
However, if you wish to take your time and kill enemies, then the Scout is much more difficult. It has some of the weakest weapons in the game, which means that it'll be difficult to kill anything stronger than an Orge with. Hit and run tactics are highly recommended when using this class to battle enemies. The Nailgun should be your primary weapon, as it deals more damage than the Single Shotgun and its ammo, Nails, is common. If you need to kill a target from long range and the target is fast, use the Single Shotgun. While it deals less damage than the Nailgun, its shots will instantly hit the enemy.
The Sniper is one of the easiest classes to use in Co-op. The Sniper Rifle can kill anything weaker than a Fiend in one shot if it is fully charged up. However, it takes time for the Sniper Rifle to be charged up, so the best way to use the Sniper Rifle is to see an enemy, get out of its view, charge the Sniper Rifle, then run back in and fire.
The Automatic Rifle can be useful for taking out the enemies in the first map of an episode, but its weak power makes it almost useless past that point because of how little damage it does per shot. It can be used to finish off enemies that've been injured by a Sniper Rifle shot, but the Nailgun does more damage and doesn't use up Shells.
As in other game modes, the Soldier is one of the most useful classes in Co-op. The Rocket Launcher can be used to skip parts of almost every map, and it packs a punch to boot. Most enemies will be killed by two to three rockets, but some toughies, such as the Fiend and Vore, will need about four or so. The only downside of the Rocket Launcher is that it has a 4-round magazine, so reload whenever you can.
The Super Shotgun can be a useful weapon if you are almost out of Rockets or wish to preserve them. Shells are common, so there is no need to worry about running out of them, especially since the Soldier can hold up to 100 at once.
In Co-op, the Demoman is a good choice for those that like blowing things up like a Soldier, but want a more challenging experience. The Grenade Launcher is a powerful weapon, but making direct impacts is the only way to reliable damage enemies. The explosion made by a grenade exploding on the ground is not nearly as effective as a direct impact.
The Pipebomb Launcher can be very useful for setting up traps near enemies that you know are going to appear soon. About five or so Pipebombs should be placed near at area to kill enemies as strong as a Fiend with one detonation. If an enemy survives a Pipe trap, then use the Single Shotgun to finish him off.
As with the Soldier's Rocket Launcher, the Pipebomb Launcher can be used to skip portions of many maps. However, keep in mind that Pipebombs do a lot of damage to you if you get near one of them exploding, so do not use more than one when trying to Pipebomb jump. Otherwise, you'll likely gib yourself.
Difficulty: Easy to Medium
As with the Scout, the class' difficulty changes depending on how you want to play through an episode. If you want to run though an episode as fast as you can, the Medic is a good choice, as it is fast, can Concjump and has weapons powerful enough to take out most that get in your way without too much difficulty.
If you want to go through an episode while killing most of the enemies, the class becomes more difficult. The Medic's weaponry is powerful, but it eats up a lot of ammo quickly, especially the Super Nailgun. A good way to preserve Nails is to use the Super Shotgun to pick off weaker enemies, such as Ogres. That way, when you encounter a strong enemy, such as a Fiend, you'll have as many Nails as you need to take it out. However, the Super Shotgun can use a lot of Shells in no time as well, so always be on the lookout for ammo and make sure that all of your shots count.
Note that one of the Medic's weapons can kill Zombies, so try to run past them whenever you can. If you can't, knock them down, then run past them.
Don't forget to take advantage of the Medic' health regen ability. Instead of taking Medkits when you have most of your health, let your regen ability heal you up. That way, you can keep Medkits for occasions when you really need them.
The Heavy is surprisingly one of the more difficult classes to play as. His Minigun can only be fired while standing still. In Quake, enemies can quickly do a lot of damage if you stand still. While the enemy(ies) will take a lot of damage form the Minigun, so will you. Having to find Medkits after fighting of most enemies can be frustrating and can quickly use up resources in the maps, so it's generally not a good idea to use the Minigun. Since the Assault Cannon (minigun) uses cells to power the motor, you can't use it frequently. That, its limited mobility, and the ammo consumption make HWGuy a hard class to play in this case.
The best way to play as the Heavy is to run around with the Super Shotgun. You generally don't have to worry about running out of Shells, as the Heavy can hold up to 200. In addition, Shells are the most common ammo type in all of the episodes. The Super Shotgun packs a punch as well, so you usually don't have to worry that you'll be firing a lot of shots at most enemies. Obviously, the most valuable part of the SSG is that you can run around while firing, which makes it means that you'll be absorbing much less damage than you would while using the Minigun.
The Pyro can be a fun class to use if utilized properly. While he starts off with a large amount of Cells for the Flamethrower, most of the maps contain very few Cells. In addition, his Incendiary Cannon uses three Rockets per shot and the Single Shotgun is weak. To get over this, you'll need to exploit how the game handles an AI-controlled enemy behaves while on fire.
While an enemy is on fire, he will constantly perform his "stunned" animation. This gives you a great opportunity to either run away or attack him with the Single Shotgun. In fact, the easiest way to play as the Pyro is to attack an enemy with the Flamethrower long enough so that you set the enemy on fire, then either run away or finish him off. Mastering this makes the Pyro an easy class to play with, but remember that you can sometimes avoid enemies without having to stun them first. This can be very useful for conserving Cells, as most maps have few to no boxes of Cells to pick up.
The Spy is one of the most difficult classes to play as in Co-op. Disguises do not work on monsters, it is very difficult to backstab an enemy with the Knife, the Tranquilizer Gun does not affect monsters, has no weapons that can gib a Zombie outside of the Frag Grenade, and the Spy can carry a small amount of Shells and medium amount of Nails.
The best way to play as the Spy is to use the Nailgun as your primary weapon. While Nails aren't as common as Shells, using the Nailagun will allow you to conserve Shells so that you won't have to constantly be picking up shell boxes, especially while you're trying to fight off a strong enemy.
When playing as the Spy, try to avoid as many enemies as you can. The less ammo you use, the better off you'll be when you face off a stronger foe.
The Enginner is a fun class to play in Co-op. The Sentry Gun can tear up enemies in no time, as enemies will not try to destroy it and it can be upgraded. Because you can build a Sentry as many times as you want, all you need to do is simply blow up a Sentry as soon as all of the enemies in an area are killed.
When playing as the Engineer, always remember to build a Dispenser soon after you start a map. Sentry Guns are very effective in Co-op, but Cells are very hard to find in almost all of the single player Quake maps. Dispensers automatically generate Cells over time, which is vital for building Sentry Guns multiple times in a map.
While traveling between areas, have the Railgun out. It doesn't use Shells, which should be preserved for Sentry use. Its fast rate of fire allows it to inflict a decent amount of damage in a short amount of time.