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...