I also weakened the doctor, he had -50% everything. And I reloaded to a point before talking to him for the first time several times, restarting GOG and everything; but every single time he transforms into demon form as soon as the battle starts... is that supposed to happen?
He always goes first and transforms, then he proceeds to use scrolls and potions. I don't buy his stuff before the battle starts... It's also worth noting that all of the times I killed him except one (the last time I killed him), his entire vendor inventory drops =0