Regardless of how durable a body is, with enough use and beating, it can/will be worn down - Like a rut that is formed over years of travel.
Warden may win through literally eroding Ultron's body. As one warden shell goes down, he returns with the next body, for as long as he has to until Ultron is but dust.
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Ultron can still switch bodies and make new ones
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How would he do that while fighting the Warden? He can't.
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Why do you think the avengers have had such a hard time killing ultron? He always switches bodies.
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Yes, but he's always made the body before the battle, or had backup plans. If the Warden, a 1000 000 year old promethean, were to materialize in front of Ultron, he would have literally no idea what to do, and would be unprepared.
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One must always assume ultron has backup bodies, and if ultron materialized in front of warden it would be the same
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Save for the fact that Warden always has millions of bodies. He has the collective data of everything that has happened in the last 1 000 000 years. He would, if they coexisted in the same universe, have data on Ultron. All his victories, all his loses. If it was a first time encounter, I suppose warden could always use slip space to send Ultron into the sun.