yeah, it would probably just be banned from the everything period.
if for some reason they ever decided to make a pokemon like that, the only way it could possibly be considered not broken is if the move pool it has is like 4 really really crappy moves, and even then its still not likely to ever lose because you kinda have to know in advance to prep for fighting it.
How much health does it have? Because I figure any Pokemon with Wonder Guard should have only 1 hp, in which case any Toxic, Will-o-Wisp, or any other round-about method of killing it would prolly keep it alright.
If it was Dark/Ghost, had WG, AND insanely high hp, it could just be generally OP even for Ubers.
There's nothing that sucks worse that that moment in an argument when you realize that you are wrong.