It's not going to be a fight.

The only reason that Souichirou was able to kill Medusa in the Unlimited Blade Works route was because he attacked her by surprise when she was not aware of him.

In a one-on-one fight, she would be aware of him, so she could just use Cybele or even Bellerophon to win this.

Moreover, unlike Saber (Artoria), Medusa normally fights with ranged attacks, so it's going to be hard for Souichirou to even attack her.

There’s no need for Noble Phantasms. If Medusa was not restricted by Shinji as a master she’s easily one of the top 3 servants ever summoned in Fuyuki. It’s Medusa, tf is a highschool teacher gonna do?

Souichirou was at best able to handle Saber at her weakest thanks to his fighting style being good during the first encounter. Sakura powered Medusa was able to challenge the VASTLY more powerful Saber Alter for an extended period of time.

Sou literally no diffed Saber with Caster's magic AMP in 2006 who is stronger than Medusa by a mile.

Wow. You're really new to the verse huh?

Saber under Shirou is stronger than Medusa under Shinji, but you have clearly marked Medusa as under Sakura, and while Saber is quite weakened under Shirou, Medusa is just even worse off when under Shinji's useless ass.

As I previously stated, at her full power as Sakura's servant, Medusa was able to handle fighting the drastically more powerful Saber Alter for an extended period of time. This isn't up for debate, it's just a thing that happens in Heaven's Feel.

Souichrou only did so well because Snake is a style specialized in catching opponents off guard the first fight, and Saber didn't expect him to be able to do anything because without the amp he wouldn't. Not to mention the amp provides no protection from Medusa's Eyes, no means of handling Bellerophon, it just makes him punch harder.