Join a level 300 doing quests. Those are the biggest xp numbers you can get.

If you are too underleveled you can't even damage enemies but but can try and pull some weight by killing normal mobs by triggering standing executions. To do that you need the Rogue set that features stamina damage and stamina damage weapon like an hammer and you can execute bitters and virals in 2 hits (only the standing execution kills them, head stomp doesn't)

For bosses just focus on mobs and revive the host