Maybe you're not supposed to win the fight. Maybe Terra, Aqua, and Ven lose. We don't even know when that fight takes place or what the circumstances were. Master Xehanort could have just appeared out of nowhere after VAT finished fighting off a bagillion and some "enemies" and had therefore exhausted much of their energy before he even showed up. I doubt even Sora could have beaten Sephiroth had he decided to show up and fight right after the thousand Heartless battle (if that battle had been nearly as tough as it looked like it was going to be in the original trailer).
Which brings me to my next point. Square had to create an interesting fight for the Secret Ending, but that doesn't mean the actual battle will even slightly resemble it. Just like the thousand Heartless battle, which I distinctly remember seeing a couple of Behemoths, numerous Wyverns, and several other at least slightly difficult Heartless included in when I watched an official trailer, and yet ended up being nothing but a bunch of Armored Knights and Surveillance Robots, the SE fight is probably just a theatrical treat for the people who met the requirements in the game. Especially if MX is a mandatory boss fight, I doubt they'll make him as hard as Sephiroth.