If we believe the words of Glorfindel, then yes Bombadil can be beaten.
Quote:
Council of Elrond
"But in any case" said Glorfindel, "to send the Ring to him would only postpone the day of evil. He is far away. We could not now take it back to him, unguessed, unmarked by and spy. And even if we could, soon or late the Lord of the Rings would learn of its hiding place and would bend all his power towards it. Could that power be defied by bombadil alone? I think not. I think that in the end, if all else is conquered, Bombadil will fall, Last as he was First; and then night will come."
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So you got to ask if you believe Glorfindel's words, because this is only his speculation. What HE thinks, and HE thinks Bombadil would die/fall. I don't see any reason not to believe Glorfindel, he is one of the more powerful elves left on Middle-Earth, and grew in wisdom with his death an "reincarnation."