If the 8"that you want to remove is not naturally at the join point then, assuming that you have a smallish tenon(rectangular shaped blade)saw, it might be better to make 2 vertical cuts the 8"apart and ease that bit of skirting out. It is a bit of a slow process but probably wont cause as much damage as leavering off a long bit of skirting. You may also get lucky in that the nails that the skirting is held in with may not be within that cut piece.
You could be really crude and drill/chisel just inside the edges of the 8" piece and tidy up the remaining edges afterwards.
You will still of course need to be careful in getting the cut piece of the wall in the way that the previous contributors have suggested.
If by any chance the nail position is obvious(say indentation in skirting) and it is within the 8" piece then I would recommend that you crudely expose it so that you can lever it out with a claw of some kind.
You won't be using the off-cut so it doesn't matter what damage you do to it does it.