https://steamproxy.net/app/356190/discussions/0/1727575977544812735/

I posted this on the forums, decided to add a link to my post here as a guide.

TLDR: Find an orc that's intimidated by another orc, level him to max level, and if he retains that intimidated perk, have his intimidator constantly ambush him over and over. Every time he retreats when the battle starts, you'll receive 10k exp.

With an EXP booster, this jumps to 20k EXP.

Very easy to do. I went from level 50 to level 55 in about 20 minutes.