Soldier
As the heavy hitter of the team it is advisable to stay in front of the other classes and let them support you. Fire the minigun in two-second bursts to maintain accuracy. Press "R2" to activate your force field and absorb all of the damage the enemy throws at you. The shielded area starts off small, but as you level up, the shield protects a larger area. Stand in front of teammates with the shield active to protect them, but be aware that your shield can only be replenished by a spec-ops.
Spec-Ops
Stay close to your teammates, as this class is the most vulnerable to damage, but is an essential support class. Press "O" to deploy ammunition crates to give the soldiers and medics the food and supplies they need. Stay close to a soldier to be protected by his shield, and throw an ammunition box when you notice the shield fade. Aim slightly upwards when deploying ammunition boxes to throw it further if there is distance between you and your teammates. The "Static Charge" secondary fire on your rifle gets an extra shot with each ammunition crate you deploy.
Medic
Use the medic gun to drain health from enemies from a distance. The primary role of the medic is to keep the rest of the team alive. Aim at a teammate and press "R2" when holding the medic gun to launch healing energy at him. This is recharged by attacking enemies with the medic gun's primary fire, or by collecting ammunition dropped by the spec-ops. Stay away from the front line because you risk stealing ammunition intended for the soldier, who may require it at that instant to protect the team.
General Tips
Move with the team and do not run off by yourself. A powerful formation of classes that utilizes everyone's abilities starts with one or two soldiers at the front of the squad, with a spec-ops behind them deploying ammunition and medics bringing up the rear. The soldiers should be able to maintain a constant shield that will protect everyone behind them with supplies from the spec-ops. When enemy damage does get past the shields, the medics keep everyone healthy. Should a teammate fall, the medics can move up to his position and revive him quickly.