The problem, as far as I can tell, is that a spacetime distortion/implosion is indiscriminate in its damage-dealing. We'd probably get Herakles (if it is Herakles), but we'd also get the entire city.
As for Medusa, we don't know who she is IC... but Archer does, and he has Harpe. It's the weapon that killed her, and so it'll be particularly effective. We should have Archer fight Medusa, but then we still have to deal with Einzbern, Precia, and Berserker... argh. I'm too tired to think. Just throwing this out there.