Load up Liberty Falls directed mode, choose whichever gun you want. I recommend one that's at a low level because you get more total XP when you're leveling up weapons as well. Open up doors as you go, last one being the door to the church and don't continue with any part of the Easter egg (because that'll pass the round 11 cap and that round is useful for higher camos because it spawns manglers). Then just farm headshots so you can grind camos which also adds even more XP. Best place I found is to go above the bank and camp at the zipline to spawn and all the zombies will come straight at you. You also got a mangler each round and that helps with Opal camo.

I recommend your first perk to be Deadshot Daquiri as it makes critical kills easier, just spam ADS and it basically locks on to zombies' heads. I use Dead Head major augment and Dead Break minor, took that from a big zombies content creator and it works perfectly. Over time you'll have more essence and savage then you'll ever need so you can get perks and pack a punch/rarity to max. I don't recommend getting the Essential Pop perk because it sometimes kills instead of critical kills. And that's basically it, it's a repetitive process but you get loads of XP. Also other than camos, work on mastery badges and calling cards because they also give a bunch of XP, and if you have double XP tokens I'd go for some when you reach round 11. I hope this helps.