Mandy wins, obviously. She'll have Grim wrapped around her finger for all eternity. As Havoc said, the mercenary should have no relation to any of the parties, otherwise bias becomes a factor. So, if you take into consideration an unbiased mercenary, then the winner will be whoever benefits that mercenary's own personal agenda the most. Say the mercenary was Dark Danny, for instance. He has no loyalties to these three. Yes, he did have a one night stand with Mandy, but he'd still kill her if it benefited him. We last saw that Dark Danny wanted to "whip himself into shape" so he'd most likely side with whoever promised him power. I think Aku would be the most capable of this, unless Mandy has some kind of technology she could use like when she de-powered Mimi. With a different mercenary, though, the answer and reasoning will change.