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.

@LorenerdSupreme

You forgot that her goal at the time was to take control of Saber (Artoria), not to kill her.

My point is something like that: Take the fight between Bazett against Shirou and Saber (Artoria). If Medea uses a high number of the Dragon Tooth Warriors, Saber (Artoria) will eventually become exhausted. Medea has more mana than Saber (Artoria). When she is under Shirou, she does not need to eat or sleep, and she can attack from far away. Take the fight Achilles against the golems of Avicebron, for example, Avicebron succeeded in stopping Achilles' chariot with his golems.

Avicebron's golems are drastically stronger than the Dragon Tooth Warriors, the golems could take a couple hits from fully powered Mordred, Shirou could destroy the Dragon Tooths with a reinforced object. If that strategy was viable she wouldn't have been explicitly called the weakest servant in the war, nor Casters the weakest class of them all.

My point is unchanged and unchallenged, Casters, as a class, get the shortest possible end of the stick in Grail Wars. Almost every Saber can no-sell their entire arsenal, as can many Archers, Lancers, and Riders. Their Magic/Magecraft is what they are relying on 99 times out of a hundred, at best they can huddle up in a temple they make and do their best to hold enemies off. Especially since Medea is one of the best Casters, there are loads of weaker ones with worse Territory Creation and Item Construction and lower tier spells and either worse or no summons to run interference. Everyone was a serious problem for Medea. Even if she had a proper master that would hardly change.