There are five unique classes for each side. These classes complement each other—GDF Medics and Strogg Technicians provide valuable health (as well as ammo for the technician) and can revive teammates, GDF Soldiers and Strogg Aggressors provide support through heavy weaponry, and GDF Field Ops and Strogg Oppressors offer long-range fire support through their deployables.
Map objectives require certain classes—the soldiers and aggressors can destroy objectives, the covert ops and infiltrators hack objectives, and the engineers and constructors build objectives. A team can't succeed without a diverse roster. A team full of covert ops or infiltrators using their sniper rifles and railguns won't last long against a mixed squad. If the map objective is to build a bridge and your team doesn't have an engineer then you're in a no-win situation and you may as well be playing deathmatch instead of a team game.
Everything works in unison, including vehicle usage. Engineers or constructors should provide support with their repair tools. Soldiers and aggressors can help escort vehicles and eliminate anti-vehicle threats such as enemy soldiers or aggressors armed with rocket launchers or obliterators or by destroying anti-vehicle turrets. And vice versa, utilize your vehicles to provide support for ground forces. Titans or Desecrators should bombard anti-personnel turrets to help ground troops. If you're simply the driver, switch into a vehicle's turret for additional firepower. On the other hand, you shouldn't drive off in a vehicle without seeking out a passenger.
Even with the best teamwork possible, there's still a twitch aspect to the game. You must still become proficient with weaponry. Skillful aiming is extremely important because you will inflict greater damage (twice as much damage, in fact) when your weapons fire strikes an enemy head and less when your projectiles strike an enemy's legs. Players that consistently aim for the head will have more kills and fewer deaths than players who fail to emphasize aiming. Crouching or going prone will increase the accuracy of your shots; to decrease weapon spread, use short, controlled bursts, particularly with the GDF assault rifle and Strogg lacerator.
"Learn the maps" is a cliche tip for first-person shooters but it's even more important for Quake Wars. Not only should you learn the terrain, cover, and general layout of each map, but since each map feature unique objectives, it's extremely important to understand the location of all primary and secondary objectives. This is vital for both offensive and defensive tasks. Learn the quickest routes to objectives so you can maintain pressure on offense or aid your teammates on defense. Toggle through available missions to mark the location of primary and optional tasks.
The map objectives help determine your team's class roster. If you're playing as GDF on Valley, the first task is to build a bridge with a GDF Engineer. If your team lacks an engineer, obviously it's going to be impossible to progress to the next objective, much less actually win the map. Make changes as necessary depending on the current map and objective.
For the first section of Valley, you will need more engineers to complete the bridge task and to repair the MCP as it moves into position to complete the second task. But with those tasks completed, engineers become less important—at least as far as map objectives go. Valley's next objectives require a covert ops to hack and a soldier to destroy. For the final push, a few players could switch to these classes so you have more players able to complete the objective. A single covert ops will have more trouble than a group.
There's a downside to switching classes midgame (or during a three-map campaign game). As you play a particular class, you build experience—for instance, an engineer gains experience from repairs and a medic gain experience from reviving teammates. Gaining experience unlocks class rewards; there are four rewards for each class. Plus since classes complement one another, it's still important to have an engineer around to repair vehicles or a medic around to heal troops.
It takes time to reach the higher rewards and these are reset once a three-map campaign game is complete. If you switch classes, the previous class retains its rewards but your new class' experience begins from wherever it left off—which could be zero. Before switching classes, assess what will be more important: your class' unlocked rewards or the addition of the new class to your current team roster. Often when you're trying to make the final push to complete the map's last objective, it can be more important to have enough of the required class type to reach and complete that last task and win the map.
Like "knowing the map" it's also extremely important to understand each class' unique abilities. Check the GDF and Strogg class sections of this game guide to learn each class' diverse abilities (and experience rewards) and how to utilize each class in specific situations. Understand as a GDF Covert Ops or Strogg Infiltrator it's your job to provide radar for your team; know as a Strogg Technician, you should create spawn hosts to help your Strogg teammates return to battle as quickly as possible; and remember as a GDF Field Ops, one of your primary duties is to drop ammo packs to keep teammates supplied.
Check your team's class roster and communicate to your teammates the team needs—perhaps the team needs more engineers or soldiers to aid in the current task. After firefights, communicate to GDF Medics or Strogg Technicians and request medpacks or Stroyent cells. Or point out the location of wounded teammates for these players to assist.