Skinsaw man is no match for the PCs on his own, I know from experience and also from other GMs. Like always, I think the best choice is to add minions in his room, indeed something like one or several goblin ghast assistants. I don't know how he is supposed to get sneak attack damage as written, maybe by feinting or something?
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Wether or not Skinsaw Man is a challenge depends on a lot of factors. I think the intent is that the manor is taken on all at once but there is nothing from stopping a group from leaving and coming back and even tho haunts may reset once they know about them haunts become less of a threat. The Skinsaw Man by himself is only a CR 7 for what should be a 6th level party so not very challenging unless the party isn't optimized and only 15 point buy.
When I ran it I added more goblin geists mostly to tie up the partys heavy melee dps.
Skinsaw man should especially with the help of the goblins heavily abuse the bottleneck of the room door. Also he has a great acrobatics score and should use it to move into better position and avoid aoo's. Getting sneak attack isn't as big as landing paralysis from ghoul fever. The same should be said of the Goblin Ghasts. They are low CR and should maximize their impact by making sure they get full attacks so they should ideally only 5ft step to move and lure the PC's to them. In fact it benefits them greatly to lure the party to The Skinsaw Man, they have 17 int and plenty smart enough to scheme. Just because they are ravenous hungry undead doesn't mean they are dumb. Also once things move into the room do not forget Vorel's legacy haunt. The will save is pretty low but a bad dice roll could take a PC out of the fight.
I think you should be satisfied if Skinsaw man can get a few attacks on his obsession but he isn't the boss of the book so he doesn't need to be super hard. He is better as a creepy threat that should stick in the partys mind. Maybe even make them leary of future fans.