Dead Island Definitive Edition

A good way to farm this is to wait until you're a high level (20+) and do Arena A in the Bloodbath Arena DLC. The zombies will die in one shot, so you shouldn't have a problem getting kills. When you run out of ammo, just leave the arena and buy some more from the vendor in the arena lobby.

A free way of getting this, although slightly slow, is to play as Purna and buy the Rage skill early on (I bought mine as the 2nd skill upgrade) and then use it as soon as you find a cluster of zombies (4+), although you can always use the skill on smaller amounts as long as you can easily build your rage up again.

It took me until about Chapter 5 to get it, with less than 10 kills before I started her play-through, so a good few hours of game-play, but as long as you're playing her to get Right 4 Life, you can keep on using Rage to get towards the total.