Leaving his enemy dead upon the floor the oldest brother proceeded to an inn, where he boasted loudly of the powerful wand he had snatched from Death himself, and of how it made him invincible.
Invincible masters of death, Grindelwald and Dumbledore!
He believes that the Elder Wand removes his last weakness and makes him truly invincible.
Now he came to think about it, he had not tried casting a Patronus with the blackthorn wand…He must try that in the morning… If only there was a way of getting a better wand… And desire for the Elder Wand, the Deathstick, unbeatable, invincible, swallowed him once more… They packed up the tent next morning and moved on through a dreary shower of rain.
I thought that, if we ever found it, it might be useful in hiding Ariana, but our interest in the Cloak was mainly that it completed the trio, for the legend said that the man who had united all three objects would then be truly master of death, which we took to mean ’invincible.’
There are no more uses of "invincible" in the book.