Gandalf vs. Sauron (Hobbit Movie Spoilers)

[Legate of Amon Lanc]

1) Personal opinion: theoretically, I don't see a reason why Gandalf couldn't use his full power (or, as much power as his mortal incarnation allowed him) if facing such a challenge. It would not be entirely appropriate, but he could go as far as he had gone with Balrog - but then, just like with Balrog, he might have died. Only after that it would be up to Eru to reconsider whether to send him back to finish his task.

Personally, I don't think it so unimaginable: let us imagine that really Gandalf would have encountered and dueled Sauron in Dol Guldur. Sauron could not be totally annihilated - there was the Ring still existing somewhere - but he could be defeated and in such case, he would flee and have to recuperate for longer time (much longer than he had to in the "real historical" scenario where he merely fled from Dol Guldur to rebuild his strength in Mordor). Given that, Gandalf might be allowed to return (especially if the treachery of Saruman and lack of good "leading figure" among the Istari would be already foreseen by Eru or the Powers) in order to be around to finish off Sauron later on, when the Ring would finally be found.

2) Also personal opinion - I believe yes, under certain circumstances. It is clear that Sauron was not equally strong all the time, and let us remember, he was actually hiding in Dol Guldur (that much PJ did right... even though in a bit awkward way). Also, after all, Sauron eventually fled from Dol Guldur in the face of White Council's assault - it remains questionable whether Sauron would have acted differently were the battle with Gandalf alone, as opposed to the prospect of battling the whole Council, which seemed to be too much for him (but then again, we have to take into account he certainly wasn't alone in Dol Guldur, there were at least two Ringwraiths from what the UT seems to imply, and one can guess on some armies of Orcs etc., then again nobody knows what forces did really the Council have... a long time ago, I had started a thread on this topic, I believe, which can probably still be found in case of interest).

I would say that at least in the "Dol Guldur-phase", Sauron would have felt vulnerable and it would have been, the way I imagine it, even possible for Gandalf to defeat him. After all, they were both Maiar, strong, Gandalf's power was largely diminished by his mortal form, but then again, Sauron was weakened and without the Ring; and after all, we know that Gandalf faced a quite "advanced" Maia (Durin's Bane) and defeated him. That's not to say anything about Gandalf's possible injuries/death, of course, just like with the Balrog. I believe we can also conclude a lot from the fact that Sauron was hiding from the Wise, but also Gandalf was (unlike in PJ's movie) actually obviously intentionally avoiding confronting Sauron after discovering his identity in Dol Guldur; so we can imagine the powers of the two might have been pretty balanced: if Gandalf e.g. had clear superiority, we could imagine him facing Sauron in Dol Guldur upon the discovery who the Necromancer really was (though it also doesn't feel much like Gandalf to make such rash moves, but the issue of the balance of power would certainly be a factor as well - too risky for both sides to try a full confrontation at that point).