Such a hilarious achievement that is rather easy to get. The first level you'll be forced to play is at the Battle of Little Big Horn. There are native americans on horses. Go up to one of these horses that is running around and punch it. Even though the achievement text says "in the face", I actually died a plethora of times trying to literally punch it in the face. Instead just punch it wherever and it will pretty much go down.
If you missed this one on your playthrough, it's not a big deal since it's the first level.
Theoretically you could get it on other levels as well (there are more horses out there), but since this level is a tutorial of sorts, it's a bit easier.