the light bit makes sense, but even then if you drop the amount of oxygen in air it'd be pretty fatal. i bring that one up because it will be what gets thrown around in these contests. the basic problem mpfc (and anything like it for that matter) generally has is that people will look for how can this character win. they'll stick with being in character too.
the most extreme case of this is the flash, who does have the powers and has used them against certain enemies that are extremely dangerous and a 'must win' situation. of course without the 'fight to win' clause which really doesn't make much sense with everyone, the result will... be mostly the same. that clause is what people generally want to do, and if the means are available within practical characters, the answer would be to shut down the enemy.
so breathing would be an easy focus. thus, air. air has a general composition to it with the amount of oxygen, nitrogen, and other elements in it; if oxygen dropped down to 10% instead of its usual 20-30 range it'd be pretty fatal, especially if the remains became nitrogen, and especially if it was instantaneous. i'm just pointing out here where an argument you probably don't want to happen will occur because it makes everything else cool about the power go to waste, much like Yukari's being able to switch boundaries; it can be used for a lot of creative things and is, but in mpfc the ones that matter end up being along the lines of life/death.
i mean it can still run into problems with the whole active bit (even if it's not being fundamentally changed, air is active), but yeah. i also assume the more conceptual side of things like time aren't really in her league (time is 'active'). i don't know how you've written her out in other stories or ever thought of this, but it's pretty much the main thing that'd come up.
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"Maybe it's a tentacle, molesting the planet itself. - Aschen Brodel.