Alright sorry for the late post, but I was on vacation
. Anyway, My idea for a boss would be one of the professors that was working on the virus was experimenting with one that was extremely unstable. He could not get anyone to be a test subject so he decided to use himself. At first the virus didn't affect him, but over time he began to mutate, and become insane. The Virus killed off many brain cells, and now he is on a rampage. He is immune to almost every weapon, and the only way to kill him would to be pierce his stone skin with a stake through his heart (original ain't it
). Explosions, and such render him unconscious for a little while, this could be used to stop him long enough to get the spike through him.
Well there is my idea, you can use it if ya want, tweak it if ya need, and thanks for letting me share it