When I think of a witch, I usually think of a Hag, so maybe you can reference your custom witch against level appropriate Hags to use as a baseline.
In regards to the Fighter potentially critting and taking the witch down to half health, that's a feature, not a bug. The adventurers should get to feel occasionally powerful and sometimes that's going to happen on our planned villains. Instead of trying to fight against the players by inflating mechanics or blocking their abilities, maybe find more narrative ways to escalate stakes in the event that they punt a planned boss. For example, Hags usually work in covens of three...
First off, thanks for the hag recommendation, that looks very helpful. I hadn't even thought of that connection.
Second, in regards to the fighter- I do not consider increasing the hp lightly or simply to screw over my player. My party runs hard fights and easy stomp fests, but this one is set up to be a doozy. I'm not trying to write an essay on our table, but I'm trying to set this up to be enjoyable for everyone, and they're looking for a deadly encounter here.
Does the GMG really say you should use the PC rules for this? I thought it said you should almost always use the standard creature rules, and that's what APs etc. seem to do. "You can build NPCs just like you would any other creatures. If an NPC should work like they have a class, use the class features and feats of a suitable class to pick abilities, and look at both the class’s proficiencies and ability modifiers to determine how strong the NPC’s statistics should be" from Designing NPCs
Ok, so I just pulled out my GMG and found the creature creation rules while I was flipping through to recheck what I'd seen before. Looks like I ran right by what I actually needed. I guess I need to rebuild this guy.
NPC numbers for attack bonus, HP, AC and spell DC usually scale slightly differently than they do for PCs. What I do when I create custom NPCs is look at existing statblocks from level appropriate monsters and patchwork pieces together.