I think i know why protform transformers are easy to kill. If you think of the first to movies, some Decepticons had alternate modes of earth vehicles. As you know they vehicles they choose were built with armor and made for that type of stuff. For instance lets say, Brawl, he was a tank. Maybe that armor proteceted him from all those bullets and bombs. Also look at Blackout and bonecrusher. They were military vehicles. So that means they were equipped with all the armor they needed to take the damage with ease. Now the protoform have none of this extra armor so they're easier to destroy. Think about it. It does make some sense.