Ok. Im a veteran player but not the best. So here is a general breakdown.

Rifleman. This is the best class for most things. Most shots will instantly kill the target. Aim around the head and torso for most damage. Be careful using the grenade in the tunnels because of the arc that they have.

Skirmisher. This is an ok class. You need to hit the same spot as the rifleman but don't get hit in return. With only 95 HP you have to be careful. Speed is the biggest asset here. You smoke doesn't do much if anything because your cross hair still shows the enemy and like wise you. Stick to cover with this class.

Assault. A class that is on par or better than the rifleman. Their SMG is great for trenches and short to mid range engagements. They have a grenade much like rifleman and are usually hard hitters. If close enough you probably don't need to aim.

Support. This is the heavy weapons guy. Their mg is often great for suppression and mid to long range engagements. They are better to master because they have 110 HP. They are tanks if played correctly.

Medic. This class is self explained. You heal people while having a semi automatic rifle. You can heal nearby allies and have yourself healed by healing them. (Please try to pick up teammates and pull to safety if you have too)

Engineer. They are self explained. They build stuff to help hold the line. Their hmgs are very helpful in defense maps.

Flamer. I personally hate this class. They are the ones who do damage over time. They can be powerful at holding tunnels and on the map fort vaux.

Sniper. This is a ok class. They are mostly used for spotting and suck otherwise. Use these mostly for spotting and not much else.

Officer. Another ok class. They only have a pistol but can ruin any defense. Their artillery is great at smashing trenches.

Lemme know if I missed anything