I put it on the floor, stand on it and hit it whith the 1/2 rattle gun on undo. If it spins i grab the angle grinder with a 1mm disc and cut the nut in half down from the top. Cold chisel will easly get the nut off. Dont spread the bolt when getting the nut off or it will be hard to get off the disc.
Heres a pic, its not real clear but it wil give you the idea.
No chance of damaging the disc this way.
If its got the bolts that have the hex in them for an allen key you can clean the holes out, put the key in the vice and try getting it undone that way. i dont bother any more. Just cut the nut.
You could use a 3mm disc too.
Even a 1mmm makes hot, so 3mm will make even more hot.
Then when you have the blades off, clean out the dish and pein out the deformation in the hole. Ive got a short bolt with a head on it thats about 2". I put that in the vice and sit the disc on it and tap away. Then clean it up with a cold chisel (if its got a folded over layer of metal that the old bolt head dug out) and a file to square up the edges of the holes so the new bolts will not rotate.
When im putting blades on i put the blade in the vice, cutting edge down. Do the bolt up with a spanner, i think its 15Nm its meant to be done up too.
I also put a little bit of oil in the cup where the blade sits, once the blade is done up and the vice holding the blade, work the disc back and forth a few times to make sure the blades swing.