I don't think Gandalf will be redirecting a Killing Curse any time soon. Istari magic isn't like Potter's. There are no energy beams "sizzling through the air" in Middle-Earth. It's more alchemy and divinity-driven magic, like Gandalf's "Flame of Arnor" and that light he used against the Nazgul. The speed and lethality that Voldemort can unleash is too much.
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I disagree. Plus Gandalf knows all the spells of men, elves and orcs. He also manges to battle Sauron in a contest of wills. He also was shown beinag able to kill goblins in a "flash" and effortlessly controlling Saruman. For all we know he could say "Voldemort, your wand is broken!"