You might think that when you're in the midst of a power struggle you need to be full of bluster and aggressive energy, and some of the time that is wise. But today, when you're stuck trying to negotiate getting your way, the worst thing would be to act too pompous or confident. Let this person know that you want to find a solution that makes both of you happy. You have no interest in bulldozing over them. With true collaboration you can find an answer.