Unfortunately, Wonder Guard and Pure Power are banned in BH. However, cheating is too easy in a Custom Game, as there is no EV cap in Custom Games - meaning players can give their Pokemon >510 EVs to provide an unfair advantage that will be really hard to catch by the people running the tour.
So unless we can come up with another solution, I think that the answer is going to have to be to ban Shedinja and Medicham for this tour.