What does Ender do to win the final battle?

There is much irony in Ender's final battle because by then he has given up.

"I don't care if I pass your test, I don't care if I follow your rules. If you can cheat, so can I ... If I break this rule, they'll never let me be a commander. It would be too dangerous. I'll never have to play a game again. And that is victory."

This is the reason that Ender decides to use the Little Doctor against the planet, not part of a grand, last-ditch effort at victory, as his audience believes. Ender's final game, in terms of war, is his last battle against the buggers. He looks at it like a game, and he cheats in it, and the cheating is what allows him to win the war.