There are 2 really good ways to boost XP without having to play 12 waves of horde over and over.
Please equip your 3 Combo Perks found in the Other Tab and try to get the Combo Boost to Gold which will start you with a 5x combo. (Combo King, Combo Extension, and Combo Boost).
**Still Works Post Patch**
Campaign Method - Mission 1, Chapter 1 - 4 Player Settings - We are looking for the Gold Medal here which is a 40 kill/combo or 16500 points. You can run right by the first group of zombies in the street, and just shoot the explosive barrels and zombies under the Radio Tower to get the 20 dismemberment challenge for an additional 1000xp. I highly recommend using the Mosin Nagant with explosive damage for this to make the challenge easier. You'll get 5,000XP for the Gold Medal plus 1000xp for the mission challenge every 4 minutes, Rinse and repeat.
**Patched - Works with Disc Copies Only**
Horde Method 1 - Into the Darkness, 4 Player Settings - Wave 1 - We are looking for only a Bronze Medal here, which is about 6,000 Score. Run to the equipment boxes and grab the Nades/Mines and kill everything that is coming over the wall with them. You should have between 9 and 12 thousand points by the end of the wave. Wave 2, either down yourself or let zombies kill you. This method is about 3000XP and 1 Wave per 2 minutes. Hit Y to Restart, Rinse and repeat.
**PATCHED 2/15/20** - The Bitter End - Hard Difficulty - Wave 1 - First of all be sure your Second Chance Perk is off. As soon as you spawn, look for the red explosive containers and blow yourself up by shooting them while standing close. Hit Y to Restart, Rinse and repeat. This is 3,000xp per minute but gives no progress to horde waves. **This has been patched 2/15/20.**