"Look out that's a Bundavark. No don't shoot at it! Sure you'll blow some big holes in it, but it won't do any good, it will just make it mad."
Bullets are a really ineffective way to attack Bundavarks and its not because the Bundavark are heavily armored. Why are bullets ineffective?
The best answers will have the creature that is the most unaffected by bullet holes with the smallest amount of hand waving.