Great suggestions bonzo, unfortunately they didnt work
Because of where it sits theres no way to get the grinder in there. Tried heating it up with a blow torch and pouring water on it but still didn't loosen it, then the nut kind of disentigrated on me so now theres no way to get it off now. I did manage to get the downpipe disconnected though farther along underneath the car so the whole thing will have to come out with the engine and then try to get it off afterwards. Theres also a sensor of some kind that connects into the downpipe that I cant get off (again rusted on solid) so I'm gonna have to just disconnect the wire from the loom I reckon and just leave the wire attached to the downpipe and try and get it off later (dont really like working under the car any more than I need to). Think I've got everything else disconnected now though so hopefully I can pull it out next weekend with the help of a friends crane