It most likely would be a fight between Many Spirit Gun shots while Foul tries to summon creatures... if Yusuke could destroy Foul in the time period before the summonings, it would go to him, if not, Foul...
EDIT: Also, isn't it within Yusuke's character to start with a melee game to see how it goes? Well, doubt he'd do as such against a guy that looks like a ghost, maybe...
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Official Pidgeot of the B/W Boards Clan
:D