I'm less worried about the molecular freezing and more concerned about the combustion. You explode now by your own molecules. I specifically defined my characters powers only working on non-living because otherwise I could just convert part of someone's body into energy and kill them. How quickly can she cause things to freeze or combust? How much at once?

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I like your second proposal. When changing something's gravity does it effect other objects around it? So say you increase the gravity of the ground under someone, will it draw other things to it or is the effect confined to the thing being targeted?