[quote=@Lugubrious]
<Snipped quote by Lmpkio>

I don't know what Star Road necessarily has to do with it. If that's relevant, please do explain. It is not at all feasible to create a Friend Heart accidentally.

You can indeed just have Geno "kill" him (or try to, anyway), though there's no known way to 'revive' a spirit that's lost its body as anything more than a Striker. Maybe it's in character for Geno, as a character centered around the idea of a higher authority, but so far I've seen a repeated theme: there being no other choice. If Geno doesn't somehow know how to save Mario, he can do other things besides just kill him. There are always other choices.
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I didn't mean like actually killing him.

But I can certainly try and at least decapacitate/knock-out Mario - perhaps by simply having Geno knock into him as they break out of the castle dungeon and then colliding into the ground or something. He wouldn't be killed, but he'd certainly be knocked the fuck out.