Dead Island Definitive Edition

Once you find a hammer & get to the lighthouse you can then travel down the winding hill road as there should be 1-3 walkers there, dont go further than the broken wire gate, once you get near the broken wire gate & you've killed all zombies, turn around & run back to the lighthouse, go through the door up to the top of the lighthouse then back through the door (im not sure this regenerates the walkers but it did for me) & back down the road, just rinse & repeat until you get 15 kills, it helps to either dodge the walkers charging hit 1st or kick the walker down with LB

Easily done. Two ways.
First attempt, Run along the beaches near to where you start when you get a hammer, Upgrade it if you wish then run along the beaches hitting zombies sat down, eating a corpse or playing dead.
I've noticed the zombies who are lying down who aren't playing dead if you hit them and a "life gauge" pops up it counts towards your kill tracker.
A couple of zombies who ran towards me were easy to kick then mash their heads in but you can run away ( not advised in case they catch up with you from behind) but just pick on all on the floor and you'll have it as easy as me.
As pointed out in the comments throwing a hammer does work, I'd take caution with this to not lose it or if you get surrounded by two infected then you are funked...

Found an easier way, Follow this guide but use a friend.
I helped a friend by running ahead and kicking any and all zombies to the ground then he used his hammer to kill them without worry about getting hit by a zombie, it's easier to pick on small groups of 1/2/3 as not to get risky.

I didn't find the other solutions posted here to be particularly helpful, and many of the Youtube videos out there weren't either, but I found one solution by a very small channel that worked perfectly and can also help you get the 10 Head Are Better Than One achievement. All credit goes to the original Youtube video uploader. I just found it and want to share it with people who, like me, didn't find the other guides helpful.

Easiest place I got this achievement was in Arena A (Easy mode) in the Bloodbath arena. All the enemies are level 14 for the first 5 or so waves, so if you're at least 25 you'll one hit them. You can kill the 4 Walkers in the first wave, then go to the door and leave, then reenter to kill them again. This is super easy at it is always 2 groups of 2, but one Walker will always be slowly walking to you. This method also works for the 10 heads are better than 1 achievement, as you can easily buy ammo from the vendor if you need to.