In Tomb Raider multiplayer, weapons, skills, characters, and other things are unlocked as one levels up.
I just saw a video explaining how to farm XP and Salvage (a sort of currency to buy certain unlockables) in a solo, private match. So for an answer, I'm also OK with editing the game files instead of needlessly grinding by myself.
What’s the most efficient way to level up in multiplayer
tomb-raider-2013
Best Answer
Apparently, it's to play a solo, private match of Rescue and grind breaking wooden crates as Solarii and completing objectives as Survivors.
Even faster with friends: http://www.ps3trophies.org/forum/tomb-raider/191333-multiplayer-600-000xp-hour-prestige-4-15-20hours-coop-method-alexstunts.html
Update: So, I've finally bitten the bullet and did some grinding via solo Rescue. Here are my notes and tips: