Isca "wins" because that's her power: to always win. Darwin's power is survival, which is not the same as winning. Given the rules, he would lose and "die" but he would also survive. He's already "died" before when he, Vulcan, Petra, and Sway first encountered Krakoa, because he lost his physical form to survive. His body converted to pure energy and bonded with Vulcan, who stayed in stasis for years.
I'm actually more curious how things would work out if you put Isca in a deathmatch against another Omega level mutant like one of the several reality warpers (could they alter reality to a point where her powers wouldn't work or would her powers pre-emptively create a situation that would prevent that from happening?) or even against a cosmic being like Galactus.