I'd just order a new soup, to be hoenst.
The idea of a massive duel could fit in anywhere, to be perfectly honest.
It could slot in somewhere in the first hundred pages, end with Harry running away with his tail between his legs, have a few hundred pages of Yoda-esque teaching from Dumbledore in a swamp somewhere in East Anglia, and then a massive duel at the end of Harry destroying Voldie once and for all... maybe even a nice little hero vs. nemesis scene on top of a waterfall ending in both characters plunging to their apparent deaths. Which would allow JK to carry on the series if she so pleased!
And on that note, I'm off for a bit. Toodles.