The only servant from the Fifth Holy Grail War that Medea can't defeat was Heracles (Saber (Artoria) is also a bad match for her, (maybe even more than Heracles); however, she can take control of her with Rule Breaker). The reason she was in trouble with EMIYA was that she had underestimated him. If she takes him more seriously, she probably could win (I don't mean she wins a head-on fight).

By the way, Medea probably won in one of the bad endings when she "Allied with Shirou".

I'm not sure Medea has any countermeasures against UBW.

@Mega gk

That's why I said that "I don't mean she wins a head-on fight".

Although she can teleport herself, so maybe she could escape from Reality Marble like Unlimited Blade Works.

Is it possible to escape from the Reality Marble? I always thought that if you get trapped in it, you can't get out until the magus cancels it. Although, Medea can teleport before it's activated. I don't think she would just stand there and watch while EMIYA chants the spell.

Actually, Medea has a canonical statement about this. In the VN she mentioned that the war is a sham and she clocked the bs that is the Fuyuki Grail War after being summoned. She also stated that she has no means to defeat Saber (who she doesn’t know is King Arthur) class servant due to their naturally high magic resistance. Hence, her plan to take out Saber’s master and steal their command spell. The one single servant she couldn’t defeat was Heracles who was paired with an insanely strong master (on top of being a literal divine spirit). She needed Saber to stand a chance to defeat Heracles who possessed the most durable Noble Phantasm in the entire war.

She didn’t know Gilgamesh was around either until her death. Nor did she know about Rin Tohsaka’s talent as a magus (all modern mages are like ants compared to what she’s capable of, even users of True Magic).

She has a comment about every single servant, hinting that she knows their abilities by spying constantly on them and their masters. Cu Chulainn she couldn’t defeat either but she wasn’t worried about him due to his master (Kirei) being who he is, she was hoping Archer would deal with him or kill each other.

She wasn’t worried about Rider either as she is aware (implying in kahoots or not) with the Matou head that he switched Rider’s master forcefully using a mystic code to Shinji, who was incompetent, which is more than a boon for her as not a single hero in greece besides perseus could defeat Medusa-pre Gorgon (she knows Rider and Heracles’ identities). And she plans to have Assassin Kojirou kill himself in every timeline or route.

She then goes on into details about her plans for winning. In a bad ending in Heavens Feel route, she stole Shirou’s arm (that was actually with Archer’s) to make a mystic code that can freely use UBW reality marble anytime she wanted like a wand, but she herself said its useless to her, but she used it to win the Holy Grail War. In the end though, there’s so many things she doesn’t know about that she couldn’t have planned for contingencies. She was too cocky in her scheming that inevitably caused her to lose (besides her affection for her master). The only way she won was because Shirou (the protagonist) kept making terrible decisions (selfish ones) that gave her the opportunity to win even slightly.

So, my theory turned out to be invalid?