Mount & Blade: Warband

I found the easiest way to get this was by defeating Sea Raiders who you can generally find on the edges of the Kingdom of Vaegirs and will often run in to the snowy regions. They will tend to run from you if you have a large enough following so it may be worth attempting this with a smaller party and they will come to you, alternatively if you have a fast movement speed across the map you can just chase them.

You have to be in the snow by the kingdom of Vageirs (pink) for this to count. There are plenty of bandits that roam around here so this isn't too bad. You can make a manual save before attacking them then keep reloading it after the battle and it will keep counting.