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.
Seiya
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another place and time, without a great divide, and we could be flying deadly high