I know exactly what you mean, mine was a right s**t to get out. In the end it just came down to pretty much breaking it off, I came to the conclusion that seeing it was broken anyway it wouldn't matter, so it was a combination of pulling and twisting until it let go.
I'm sure that if the bumper was off, and you could get at it properly it would come off easy, but that seems like a right hassle !
Just be careful of the wires obviously, as they aren't overly long, and a word of warning, mine was as hard to detach the wiring connecter as it was to release the sensor, mainly due to access, but with a bit of patience you can do it !
I think it took me about an hour and a half all told. ( an hour and twenty nine getting it off, one minute to put the new one on !! )