Ah yes, Chiron will lose if they kept fighting, he knew that so he retreated.
Sieg has siegfried body with invincibility even Karna only managed to give shallow wounds, also Mordred only wanted to test him at first, but he managed to wound her then she became more serious
Karna didn't have much achievement beside those visual breaking animation in Apocrypha because he became the antagonist and face the main character, so he need to lose. He'll be succeed in burning Sieg if Astolfo didn't bullshitly pull out Akhilleus Kosmos