It is very possible that, had Melissandre been there, the battle would have gone differently because of her numerous powers over fire and magical fire (which wildfire is, since dragons being present in the world enhances its creation). However, Stannis followed his advisors (and pride) which told him that if he won while Melissandre was present, she would always be considered the one who won the kingdom for him. Who knows? It's an intriguing possibility.
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