This thread may be useful
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Gandalf the White (and Grey) is incarnate. So, probably, while a weapon may harm the flesh that he is 'clothed' in, it won't be able to do any damage to the spirit that resides within his body.
I believe that while weapons like Anduril and Aeglos may be able to harm Gandalf's
body (I mean, hey, look what Narsil did to Sauron), there probably isn't any weapon in
Middle-Earth that can harm his actual spirit. I'm not sure if even the Valar would be able to do that; no doubt Eru could cook up something to get rid of him though
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This question raises some other fascinating possibilities for me. We saw Wormtongue kill Saruman with a dagger, yet he only injured his body, whereas his spirit was dissipated by a cold wind from the West. That seems to contradict my theory that the Valar themselves probably can't harm the
spirit of an incarnate being, though that gust may have been sent on the bidding, or by, Eru himself.
In the (abandoned) Second Prophecy of Mandos, it is predicted that Melkor will enter Valinor from the Void and will be slain by the Black Sword of Turin, Gurthang and he will finally die- i.e. his spirit will be slain. This is interesting, since the spirit of (formerly) the most powerful being in Arda can be slain by the sword of a Man - albeit that 'once it bites', it kills (don't have quote on me) - yet Anduril is seemingly unable to harm Gandalf.
Himilsillion may be on to something with the 'dark sorcery' weapons- the good cannot hurt good notion, etc., but I don't know if I totally agree with this one.
So in answer to your question- I honestly am not too sure. The canon quotes and evidence I've presented appear to show that the 'clothed body' of an incarnate being can be harmed/destroyed by physical weapons, yet the actual spirit of the Maia cannot. Then there's the Second Prophecy
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