i'm going to throw in another vote for the grinder. I've used them. It works. I've cut through krypto u-locks within less than two minutes using the plug in kind. i've never tried cordless.
Another option, if you have some sort of proof of ownership (and this is a long shot) is to contact the police and have them remove the lock for you. They have to this with abandoned bikes after a time, and they have the tools and know-how to do it.
(sorry if someone mentioned this already, i didn't take the time to read everyone's answers).
J