This is unfortunately very wrong. Punching and grinding do exactly that- affect the volume and last shape of the boot. If the OP is in a shell that is 2 shell sizes too big, then widening the boot will increase the problematic volume issue that he is already experiencing. His foot will slide around even more and his muscles will try to stop this from happening which will then cause his foot to cramp out like crazy.
And his footbed is most likely wrong too if it was done by the same boot-fitter that put him into a shell 2 sizes too big.
OP - do not waste any money on trying to make a boot that is 2 shell sizes too big work. At the very least, try to find a used boot in your size/width