I won't say it's impossible this happened, but it is highly improbable that this happened.

As often is the case, there's no citation where this information is from. Doing some googling, it is possible the person in question is the daughter of Ikeda Tsuneoki, one of Oda Nobunaga's highest ranking retainers. But she would be recorded in compiled history by her temple name as Anyōin, not her personal name, so I don't even know where her name of Sen comes from. It is not impossible that some surviving letter gives us her name, but as I can find no citation anywhere, I do not know where this information is from. Various sites in the Japanese corner of the internet do mention her engaging in combat with a team of 200 women arquebusiers, but differ in which battle she participated in: Yamazaki, Shizugatake, or Komaki-Nagakute. Interestingly I can only find English sites that says she was at Sekigahara. And of course none of them cites their sources. I am tempted to just dismiss the whole thing as fiction from video games, if I didn't know some video games do include actual trivia information like this. Even giving it the benefit of the doubt, without citation I don't know if this information was taken from contemporary sources or Edo fiction.

We actually know very little about the detailed troop count and deployment at Sekigahara. Most scholars just use a rough 60~80k on each side. Those who do troop breakdowns (the one on wiki looks like it's mostly from Bryant) take their data from the Nihon Senshi Sekigaharayaku (Japanese Military History: Sekigahara Campaign) compiled by the Imperial Japanese Army and published in 1893. And its troop breakdown (found here) is very badly done. It takes one letter of the Shimazu that says Tokugawa Ieyasu has ordered the Shimazu (then on the Tokugawa side) to mobilize three men for every 100 koku, and use that ratio and apply it to every single combatant at Sekigahara. There's no reason why we should assume every daimyō at Sekigahara mobilized and brought troops to the battlefield at the same ratio. In fact, we should probably assume they didn't. Unfortunately it doesn't seem anyone has put together a better analysis of the numbers and deployment. But even the badly-done list makes no mention of Ikeda Sen or Anyōin or anyone with 200 female arquebusiers (at least I didn't see it). At this point in history, Anyōin would be the recently-widowed wife of Nakamura Kazuuji, and her brother-in-law (her son's too young) would have brought the clan's troops to Sekigahara. It is highly improbable if not outright impossible that she would have been allowed to accompany the Nakamura troops to Sekigahara, instead of staying in Sunpu Castle or Edo with her son (why would she want to anyway?). Even if she was, 200 arquebuses is no laughing matter when the Nakamura (according to the bad list above) brought 4,350 men, meaning the clan probably only brought about 1000, at most 2000, arquebuses. Why would they let her command 200 guns?

That said, from scant mentions here and there we do know at times women participated in the defense of castles, sometimes even as combatants. Open battle though, I'm very skeptical.