Gothmog vs. Sauron

[Galadriel55]

Well, it is possible that Huan could have killed Sauron; after all, he wasn't a regular dog. However, again, I'm not really sure if Sauron was indeed killed when hir Fea left his body. In the beginning of The Sil it says that the ainurs' bodies are like clothing, and their physical form could easily be changed or discarded. No one would say (I hope) that Ulmo died when he discarded his physical appearance the one or two times that he took one. Why do we all assume that Sauron died?
On the other hand, though, Gandalf says that he went through death when his Fea left his body and then was sent back in orded to finish his mission. However, Gandalf was in a hurry every time he talked about that, and most of the time he talked to those who do not now too much about the ways of Maiar, Mandos, etc. Maybe he was just putting it in a term that would be familiar to his listeners.