Sorry, but XP “Farming” has become extinct.
However, you can still “Micro-Farm” the end of each map to gain XP until you reach the kill limit (where your XP from kills is reduced to 1 XP), or you must flee the map to stay alive.
For me, the best way to “micro-farm” maps is to use 2x Assaults and Maggie in the leader position, or to use 3x Assaults. Once walker levels get higher, you can stun a lot of walkers to hold them off with an Assault, then kill those walkers with the same Assault to get the charge ability back.
At some point I hope to be posting videos showing the best way to “Micro-Farm” each map.