Not very hard to kill. They are a small animal with a small kill zone. Hit them in the vitals or head and they die. Poor shot placement and they can be tough little buggers, they will drag their ass or guts a long way to a hole. I have killed them with everything from .22 to 375 H&H.
The one below was in the wrong place at the wrong time, I was testing out a new .338 Norma with 300 SMK's @ 2800 fps. They seem to work