Coming in hot w/ an updated 2023 guide since I had to do some digging to figure out the best way to do this these days. At first I was doing small matches w/ a 2nd controller but the XP gains were so minimal I stopped...
Now that everything is offline the quickest way I've seen to level up is to do the following
1. Open up War Games > Custom > Infinity Slayer (or similar)
2. Set the time limit to 10m - no more no less due to some XP cap (trusting source)
3. Wait out the 10m timer
You'll get between 500 - ~1200 XP (details in guide) per match.
- I also tried w/ using a second controller and getting a few kills but it was too much work and the gains were negligible
- there may be a daily cap - I stopped getting XP after a bit
credit here to "Snipe MD" on 343industries.org:
https://www.343industries.org/forum/topic/30193-how-to-earn-...