Welp.
Seiya
Battle happens two ways:
1. Seiya blitzes Urza. Urza cannot react to lightspeed attacks, so he eats whatever Seiya throws at him. Magical attack probably kills him outright, intense magical stress being responsible for a huge number of planeswalker deaths in the MTG storyline. Physical attack... probably kills him outright too, because it will apparently be strong enough to affect the "real" Urza, i.e. his immaterial consciousness. Direct attacks on the soul/consciousness being the other major cause of planeswalker deaths in the MTG storyline. In fact, the latter might be a better idea, because what counts as "intense magical stress" is really inconsistent even within MTG.
2. Urza goes first, and his first action is to warp Seiya away so he doesn't have to deal with him. Seiya returns, at which point the above happens.
...I guess if Seiya was a complete idiot or something, Urza could win, but it's looking very, very unlikely.
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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