She is the second best Soresu user of her time. She is the only master of is outside of Kenobi.
However, Kit Fisto was viewed as a threat by Grievous himself, nearly stalemated Asajj Ventress, was visibly a superior duelist than Kenobi for the beginning of the Clone Wars, and took out two MagnaGuards in few flashy spins. This would actually be a wonderful fight. The two are widely reputed for their literally astounding grace, incredible lightsaber abilities, and unmatched courage and perception. They are likely the best duelists in the Order after Obi-Wan.
Kit Fisto may be a a -practitioner- of form one (It never says he mastered it, unless I am sorely mistaken) but Luminara is a bonafide master of a defensive style that was able to allow Obi-Wan to affectively defeat opponents like Grievious and even Anakin Skywalker. While Luminara is considered to be the second best Soresu master, she's actually been practicing it longer than Obi-Wan and is much more agile and graceful. I would argue that she should be better at it, but it doesn't appear that way from what little we know.