I found another method accidentally that has not yet been mentioned. If you would like to level up relatively quickly without any human opposition, you can play EXTERMINATUS PRIVATE. Set up a game on Hab Center, Arena 1. You will encounter waves of orcs, akin to a 'horde' mode, and when you die you will capture all of the XP and level up. I was actually going for the following achievement in which you need 40,000 kills.
If you follow the guide for that achievement, you can obtain over 500 kills and, more importantly, well over 3,000 XP for every 12-15 min of game time. I really like the fact that this solution contributes to many other achievements as well, and while it can be a grind (I'm still working on it at the time of this writing) it can be broken up over several days/sessions to ease the pain. Hopefully this helps anyone still going for this achievement!