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.