Rider'd be doing a lot better than in UBW and Fate since RIn can actually supply her.
Heracles'd kill Shirou instantly from sheer energy drain.
Medea would do wonderfully with someone like Bazett to defend and support her, if we ignore the fact that that would mean Kirei has Medea, unless we assume Medea prevents that because she's smart and if stolen can just use Rule Breaker on herself.
EMIYA would probably fucking hate the being pawned off onto Shinji, but under Sakura he'd do pretty well.
Cu'd have issues if he's stuck at the gate, but all Medea's layered defenses would make it REALLY fucking painful to attack.
Artoria with Illya would be incredibly dangerous do to actually having enough energy to fuel Excalibur, Mana Burst, and would generally be the monster she's supposed to be in lore but is never actually allowed to be.