Well, before I even put on my thinking cap, you could try some of Tom Bombadil’s other names that Elrond mentions. As I recall: Orald, Iarwain Ben-Adar and Forn. Though it doesn’t seem quite cricket for those to be the correct answer, since there’s no hint of using the Elve’s name for him or the Dwarves…
Treebeard as someone suggested might also be a good bet, the part about “in the end…” referring to him releasing Saruman. If you take that last line seriously, then it would rule out Bombadil since he doesn’t even figure “in the end” of the struggle
One thing to do also is to go back to all the turns you put in answers, and double-check that you made no typo and really put in the answer you intended.
My last thought is, perhaps it refers to an earlier struggle? Have to go back and re-read the War of Wrath or some of the other conflicts to see if anyone seems to jump out.