Well.

Someone else killing it solves the problem of whether this would be Death taking a life before their time. (This is different than fighting shades in the Underworld. They're already dead and will reform again and again.) He'll continue blocking the helpmate's attacks while the yellow girl fires her very slow... and multiple adamant rails.

Yes, Thanatos knows what a railgun is but not a gun gun. Hades is weird like that.