Dying Light

This is my first solution and I'm typically not great at explaining things, but I'll do my best. Apologies if it's confusing.

All you really need for this is lots of fire crackers and medkits. Both are easy enough to obtain.

The easiest place I found to do this is on the main bridge near the water (you can probably find many places on the bridge(s) throughout the game to do this). When you're looking at your map, scroll as far to the right as you can. You should see a big line (the bridge) going into the water. This is where I was "boosting" this achievement. There are a crap ton of zombies on this part of the bridge and also a lot of holes on the sides of the bridge where you can kick them off. Just throw your firecrackers near the holes to gather them and start kicking.

Drop kicking zombies off the bridge doesn't seem to count towards the achievement (it wasn't for me). Sometimes the kills just don't seem to count, and I'm not sure why, so it may take awhile.

Fastest solution, in my opinion!

At the tower, I talked to Buckshot (sitting on a couch near the first floor player stash). His FIRST challenge is to kill 40 zombies with a baseball bat. Accept and you spawn near a broken overpass. Jump onto it and start kicking the crap out of the zombies!

If you die, you don't lose points because it's a challenge. Plus, you spawn right back at Buckshot for another chance.

i got this yesterday, i managed to get it on the large bridge that goes out over the water but i must have kicked about 50 or so of the damn thing before i realised that they weren't counting. I kicked another one off and the zombie disappeared before it hit the ground and i had no power/agility points show on the screen. it seems too me that if the zombie falls to far it disappears so i coaxed a few back towards the beginning of the bridge using fire crackers and insults right around where there is a stair case up ton the north side of the bridge. It worked like a treat. every zombie i kicked over the side made contact with the ground and didn't disappear into thin air, also i noticed i was getting points on towards power/agility bars for the zombie dying from the impact. Also when i checked by snapping the achievements sure enough it was going up with every kill. Just make sure when you do it they fall far enough to die but not so far that they vanish and you don't get the points, as it says in the achievement description "Kill 100" not just kick them off somewhere. Make sure you have the drop kick bomb perk swell, this help kick the buggers over the railing in a most satisfying manner. good night, good luck.

Don't worry about this until later in the game. In the second map old town you will be mainly going across the roof tops and staying off the streets, you will come across many lone zombies as you go from place to place, simply take a minute to check which side of the roof has a drop to street level and kick them off and continue on your way. No need for med kids and fire crackers.

Then once you have finshed the game if you still need more simply come back and grind a few more on the roof tops.

I found this to be far less annoying than going to the bridge and getting mobbed or them not counting.

NOTE: Do this during the day as there are not as many up there at night + it's more dangerous.

If you just want to get this achievement without any extra hassle, I suggest that you do this for whenever you get to the 2nd zone, instead of the slums as the rooftops are very rarely high enough for this task.

Simple go along with your daily business (side quests, main story, other achievements, etc), and just kick any zombie you see near an edge to their death. Make sure there is reasonably enough height for the zombie to die, as it may just get stunned, which does not count. Also, just in case you don’t have it: Get. The. Grappling Hook - you won’t regret it.
This should save you a bit if time, as well as have some more fun when travelling alongside Harran’s rooftops if it isn’t enough already!

Happy achievement hunting!

The absolute easiest and fastest method is the following:

1. Start a new game so that only low level zombies appear (the ones barely clothed)
2. Story Mode difficulty for the least amount of health for the zombies
3. Play the early parts of the story and collect crafting materials for around 50 firecrackers
4. Do the bridge strat that is described by the other guides here
(meaning, go to the big bridge in the east of the slums, use firecrackers to lure zombies to the edge and start kicking them off)

The low difficulty and low player level ensures that the zombies die when they hit the ground. If you're not too far out on the bridge the despawning before they hit the ground won't affect you either.

This way I got my missing 45 kills in about 5 minutes.