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?

True Assassin would probably be the real problem for her, since he could easily kill her Master.

Very specific points have to be given for Caster to win the war.

Casters are, to be blunt, kinda fucked as a class. Of the 6 other classes, 4 get Magic Resistance by default. Medea was even worse because Souichirou isn't a magus and doesn't have command spells.

@LorenerdSupreme

If we're being fair, most of the Casters don't usually fight head-on, and the Magic Resistance of most of the Sabers isn't strong enough to stop the magic of a strong Caster.

The vast majority of Sabers have B or A rank Magic Resistance. While Lancers, Archers, and Riders all have a much more varied spread, Sabers are extremely top loaded. 3 have D or E, 6 have C and all of the others (that would realistically be summoned in a Holy Grail War, I didn't count summers and the like for any category) have B or A.

Not fighting in close range doesn't help when the class's whole thing is Magic/Magecraft and 4 classes have some level of negation and reduction by default.

@LorenerdSupreme

Some of the Casters can create something that can attack servants, for example, Medea can create the Dragon Tooth Warriors and send them to fight. Yes, one of them alone doesn't make much difference against a servant; however, in high numbers, they can make a difference.

That is my point when I say not in head-on fight.

The things that Shirou could destroy with a reinforced shinai? That Saber cut through hundreds of no issue? They were a speed bumb at best in large numbers, there's a reason why Medea was explicitly referred to as the weakest servant in the 5th Holy Grail War.

If Caster supporting her Dragon Tooth Warriors was at all problematic she'd have done that. And Medea is apparently in the top 5 most skilled Magus in the World too.