After completing all the levels I still hadn't gotten 50 people killed with a horse so I had to grind the rest out. The underground level (last one) is the best for doing this as a horse is always spawned in the same place with people around it. This achievement can be obtained much sooner but I found it's much more random on whether a horse will appear or not. Soon as you start just make your way down to the horse and interact with it by pressing Y. After you've gotten 3 kills with it hit start and restart to repeat the process.
Korillad makes a good point of letting the horse kick you instead of hitting restart. This will help towards the 100 deaths achievement so it would be a good idea.
Below is a video if you would like to watch.