The best way that I've discovered to earn XP, is to play a 1v1 game.
If you are the investigator, here's the best way to earn XP.
Pick up all clues
Stun the monster as often as you can
Pick up items.
I was able to earn between 1900-2300xp each run. I've almost unlocked this achievement this way. Let me know if you've discovered anything quicker.