A few questions about Gandalf not fighting Sauron

[Galadriel55]

And even if by whatever unexistent chance such assault would have succeeded, Sauron would just return again, since he lives as long as the Ring remains intact. Sure, he lived even after the destruction of the Ring because he is a Maia, but he was bereft of nearly all his power and had no means to build it back up. If the Ring is not destroyed, though, it would be a repetition of the Last Alliance - he'd eventually come back, and the battle would be useless other than for delaying the final outcome.