It's difficult to even come up with a plausible physical mechanism through which Storm could hurt Monica in light form.
One esoteric possibility is EM absorption by plasma:
https://en.wikipedia.org/wiki/Plasma_stealth#Absorption_of_EM_radiation
Specifically, if Storm could ionize the air around Monica (turn it into plasma), and set its temperature just right, so that its characteristic electron plasma frequency closely matches the frequency of Monica's light form, then Storm could in principle absorb Monica's light energy into the plasma.
https://en.wikipedia.org/wiki/Plasma_stealth#Plasma_and_its_properties
Of course, Monica could always simply switch to another frequency (assuming Storm can even guess her frequency in the first place, and create plasma with exactly the right properties to absorb it). But if Monica doesn't realize how or why she was being absorbed, Storm might actually win. Since lightning temporarily creates some plasma, simply blasting Photon with lightning bolts might actually have some effect as well (but likely not much, since it's not "fine tuned" for her frequency).
Nevertheless, Monica wins 9999/10000.