Ah yes.
Finally someone who have my same kind of ideas and thinks the Witch should be still be considered as a real threat. Very cool :3

For a Wandering Witch, yes if you aim well and don't miss a shot, she will die.
For Sitting Witches it's far more difficult. It just reduces the shotgun damage modifier for Witches, they still take normal damage (200 - 250) and therefore can still die if they are too slow to target you, such as Wandering Witches because of their long scream animation. Although they will no longer instantly die on any difficulty from a shotgun blast at pointblank range.

It's still possible if you click fast enough for auto shotguns