Here is something funny. I was playing as Davis and gave the original Davis the Hammer. Davis moved very slow and hit Pete with the Hammer. He was indestructible the whole mission until I got to the place where you throw items at him, he was indestructible with the Hammer. Try switching to Russell and give Russell his winter skin then give him the Hammer. In his boss fight he will fight completely with the Hammer and use the full boss style it is a weird glitch this works for all boss characters only for some odd reason. With normal AI's you fight in the other non boss missions they just walk and stand idle after you hit them they attack, since boss fighting AI's have specific action nodes they attack on site.