If Seiya is planeswalked into another universe, can he return?
Because that's really the deciding factor here. Seiya's probably strong enough to kill Urza outright with a magical attack (intense magical stress being one of the few things that can put down a planeswalker for good) and tanky enough to survive most of what Urza can throw at him (though his each-attack-only-works-once trick is useless, Urza's got enough spells that it doesn't matter).
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"The courtroom is the garden of holy judgment. Those with lechery in their hearts should leave this sanctuary at once!" ~Franziska von Karma