Dying Light is a new zombie game that also has a good in-game economy. If you want to easily farm XP and money in the game you can do it via the method shown in the video below.
The YouTube user also recommends an another method of leveling by “getting airdrops and doing quest which may be faster in terms of how much exp you get from finishing each quest.”
Just follow the method below. All you have to do is get on the ledge shown in the video and make some noises to lure the zombies to you and after that just use a melee weapon to smash them and gain XP.
Let us know if you have a better method to farm zombies.