I'd completely surround her castle and lay siege to it. All the while she'll keep drinking booze and pissing off her officers and servants. I will then use the river to my advantage, divert some water from it, get the sappers to make holes in her castle walls and flood the majority of its courtyard. I will then storm the castle with my troops, completely catching her by surprise and have her bound up. She'll beg for my mercy and ask to join me to conquer China together, but my ally will have none of it so we'll then execute her.
If you know what I mean of course.