Personally I don't think it's very hard at all, but the achievement is still pretty rare so maybe I'm in the minority. There's only one stage that was a real roadblock for me, and that's the bungie jumping snot boss. Until you can figure out how to consistently get him up against the wall, it's pretty hard to kill him 3x in a row without dying yourself. People seem to have trouble on the puppy love stage, but I'm guessing they don't realize you can whip him again in mid air in order to send him further. You can blow past all of the seesaws that way easily. And the final stage can kill you fast since there are spikes everywhere, but they aren't too hard to avoid. The rest is pretty easy imo.
Don't know if you can do the multi achievements locally, I did them all on XBL. The co-op missions are actually really fun so they are worth playing anyway.
