[@Prince of Seraphs]
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Except Shiro's magic is to enhance herself and her scythe, and to do short 'jumps'. Not sure how that helps in anyway with the fire.
And as stated, Manami's isn't ordinary water itself. I reckon Ash would be the best to stop this, but she hasn't been accepted yet >.>
Nor would she stop it from hitting Nidhogg...it ain't her problem and she ain't going near the dome with three high ranked Shadow Heart members there.
Manami could simply stand there with a sphere of water around of them till the fire disappears couldn't she? If the fire's fuel is 'magic' then it would have to burn itself out eventually. Her sphere's holy attributes would add some defense against the flames, and her water is magically enhanced so would be difficult to simply heat up and make it disappear.
And it can't be any more magic than Dawn himself can cast, and as I've assumed before Manami has more magic than him so in theory COULD stop it by herself, however she would be drained considerably yes but could still fight those shadow monsters if need be.

Also since it's an attack, Cody could dispel it (but won't, Nidhogg is trying to kill him :lol) if he had enough magical power to counter it. Whether he does or not is up to debate however, and if he did it would probably leave him similar to Dawn :lol

Or Georgie would simply swallow it up in darkness and be done with it. She definitely has more magic than Dawn and wouldn't be fazed by his spell coming at her at all....she loves tag!