[the phantom]

But if you re-read it, it actually says only Gandalf had a chance of defeating Sauron 1v1 with the Ring.

No. The letter reads-

Of the others only Gandalf might be expected to master him - being an emissary of the Powers and a creature of the same order, an immortal spirit taking a visible physical form. In the "Mirror of Galadriel", it appears that Galadriel conceived of herself as capable of wielding the Ring and supplanting the Dark Lord. IF so, so also were the other guardians of the Three, especially Elrond. But this is another matter. It was part of the essential deceit of the Ring to fill minds with imaginations of supreme power. But this the Great had well considered and had rejected, as is seen in Elrond's words at the Council. Galadriel's rejection of the temptation was founded upon previous thought and resolve. In any case Elrond or Galadriel would have proceeded in the policy now adopted by Sauron: they would have built up an empire with great and absolutely subservient generals and armies and engines of war, until they could challenge Sauron and destroy him by force.

That's the section that discusses people's military chances against Sauron (not one on one chances), and as you can see it says "only Gandalf might be expected to master Sauron", and it also suggests that Galadriel thought she could supplant Sauron because "It was part of the essential deceit of the Ring" (ie she really couldn't).

It says what policy Elrond and Galadriel would attempt to follow but Tolkien obviously doubts their chances of success. Not to mention that destroying Sauron's armies and such still wouldn't destroy Sauron. You'd still have to destroy the Ring. The only way you would pursue the policy "use the Ring and destroy Sauron by military force" is if you believed that you would be able to remain uncorrupted and destroy the Ring when it was all over.

This, I believe, is what Boromir wanted and what I probably would've tried. That way I would be trusting in my own ability to resist the Ring rather than trusting in a hobbit to do something that was logically impossible. Sure, it would've failed, but at the time it would've seemed like a better idea than what they actually did.

Anyway, the stuff about confronting Sauron one on one is after the part that I posted above-

Confrontation of Sauron alone, unaided, self to self, was not contemplated.

Then it gives a hypothetical Gandalf vs Sauron situation, but the others (Elrond, Galadriel, etc) are not even mentioned. Their chances of success one on one are not even addressed (other than where it says it "was not contemplated", ie they didn't have enough of a chance to even try thinking about it).