Good place to farm XP: Exploration mission XP in the latest episode, and some specific episode location. You can also stay longer in any mission after you have accomplish the objective and the way to the exit is clear.
For the specific missions, if you want to be able to repeat this mission, you hit the Flee button when you're ready to quit. You won't lose any experience points you've earned when you do this. The further you farm xp in the game the better it gets.
Note: when you flee the display of gain xp is wrong but you still get the xp. It will appear when you reload the game.
Episode 3 Edit
In Episode 3 there is some levels that are good for farming. This Episode is not the best to farm xp but the missions cost only 3 gas and are early in the game.
Episode 3.3, 3.6 and 3.7 have walkers that appear every turn. They can be killed to get xp (eventualy you will have to leave because walkers will be higher level than you).
Episode 3.4 is better. In this level, you have to close 3 doors to stop walkers from coming into a room. If you don't close the doors, you can kill infinite walkers and their level stay the same (some are armored and give more xp per kill see Walkers).
Episode 4 Edit
In Episode 4 there is some levels that are good for farming. This Episode is not the best to farm xp but the missions cost only 3 gas and are early in the game.
Episode 4.4 to 4.7 have walkers that appear every turn. They can be killed to get xp (eventualy you will have to leave because walkers will be higher level than you).
Episode 7.4 Edit
Episode 7.4 (Normal, Hard & Nightmare) is the one where you can farm zombies for hours. Once you have push the containers the level is 100% safe. There is 3 walkers that spawn every turn on the other side of the containers so you can shoot them (or use overwatch).
This missions can give more xp than level 7.4. In this episode, you have to push containers to block the access to a yard. Once done, zombies will breach trough the fence on the left side of map (3 every turn) and continue to spawn on the right side behind the containers (they can't push back the containers). You can kill as many as you want there. A trick is to let a hunter on the right (somewhere between the containers ) to shoot over the container and use the 2 other survivors to kill those on the left. That way you can kill up to 6 walkers every turn. Be careful if you use only the overwatch because the level is not 100% safe like level 7.4 (they spawn in the yard with you on the left).
Challenge mission - Clear the Containers Edit
in the Clear the Containers challenge mission, the walkers will be the same level for at least several hundred kills.
Bring a team of ranged survivors - ideally you want each one to be able to get a kill with each shot.
Close two containers on the same side of the screen. Bunch your survivors together in the corner next to the container you just closed. Leave them on overwatch, pressing End Turn only and make sure you keep killing every round.
Once you get bored, go close the other two containers, and end the mission.