Always choose the team who is attacking, because there you can get extra points for doing the objectives.
Also if your team wins you get 750 points, if you lose you can quickly move to the other team. But that would be a shame.

Knight/Guardian, Archer/Crossbowman - Banner
You can farm a lot of points with the banner, you have to be patient for your team to stay together. Most of the time this happens when there is an objective to capture a zone.

Archer/Skirmisher - Javelins
The javelins are the deadliest projectiles in Chivalry 2, they deal a lot of damage, even if you don't kill anyone, every time you hit someone you get points and if someone else will kill that person, you take extra points for assist.

Knight/Officer, Vanguard/Raider - Trumpet/War Horn
Like the banner, you have to be patient for your team to stay together and be sure that no one use the trumpet before you. You can get between 300-600 points if you use it at the right time.

Destroy, repair, repeat
Destroy ballistas and catapults how much you can, there are stages in every map where you can find them behind your spawn. You don't need to be afraid if you are deserting, you lose only 50 points.
Also never avoid the enemy barricades, they can be easily destroyed and give 35-70 points.

Total Points/Repair Time
Ballista: 150-185 Points - 8 Seconds
Catapult: 54-75 Points and 270-300 - 5 Seconds ( not every catapult can give points )

The points taken from the destruction of ballistas and catapults depends a lot on damage and number of hits.
For example if I just spam left click with the messer at a ballista, I get 162 points and is more slower, but if I spam wheel down I get 175 points and is more faster. But if I use a secondary weapon it will take longer and i'll get more points.
Also if you shoot with ballista any construction, you get 100 points for each shot.

The best maps
The Dark Forest because there are a lot of ballistas and catapults, so you can farm on these in almost every objective.
The Reclamation of Montcrux as attacker because you got a lot of ballistas in spawn when you reach the objective to open the portcullis and you have almost 10 minutes to farm on these if your team is losing.

Learn to shoot with the catapult, a good shot can give you a lot of points.

The Raid on Aberfell the objective to topple the pagan stones gives the most points of all objectives. Each stone can be pushed 3 times and you get 200 points each time you push it, the pushing duration is 3 seconds.

The Razing of Askandir the objective to topple the columns, same like the one from Aberfell.
The Siege Of Rudhelm the objective to push the siege towers gives 250 points if you manage to stay in a tower when you complete the objective, also only one tower reach the wall when the objective is completed, that means you still have time to go to the other tower that has not yet reached the wall and get more points.
If the objective is to capture/defend a zone you can get a lot of points if you stay in the zone and 300-350 points if you capture it.
The Sacking of Bridgetown as agathian just stay on the bridge and you get infinite points for defending.

When your objective is to burn you can get for each burning house/tent 100 points.

When your objective is to loot stuff you can get every time you loot 75 points.

When your objective is to destroy you can get 17-25 points for every hit and destruction.

If you ever see a cart with ladder pieces, just go for it, for each built ladder you get 250 points.
The Reclamation of Montcrux as attacker you have deployable ladders on the wall, you get 100 points for each ladder you deploy.

Regicide at Trayan Citadel when you breach the main gate as agathian, you will have the objective to destroy blockades with explosives barrels in the left and right sides until you reach the statue, instead destroy them with melee, this way you will make more points an it will take more time to destroy them.

Here it's more about farming gold than xp, it seems that in deathmatch servers the gold is received depending on how long you have been in the match. There are some matches that can last an hour.
Here you can see what I got after a 30 minutes match.

You need a friend for this one, when he is low you can hit him once and revive after, you can repeat the process as much as you want, you can farm 50 points every 2 seconds.

Even if you win or lose, you will have a lot of points and time that will turn into XP and GOLD.
Enjoy the PAIN