Mauser C96 – ★★★ Handgun
Some may claim her skill is useful (It pretends to be), except it applies after the penalty (and really, its' multiplication and isn't on the same modifier so its autoshit either way), so 100% - 90% = 10% x 2 = 20%! Congratulations, you've wasted 600 basic data, 1020 intermediate data, and 500 advanced data for less benefit than a 2* PEQ.
If you need some ACC for your ARs at night, just use some ACC fairies like Illumination or Rescue. If you need ACC for your RFs at night agaisnt EVA+Armor enemies, use Fairies + Tiles, or just use Type59/SPP-1, both whom gives more.
That being said, there might be an argument to use a more offensive fairy instead of an ACC one (neither Artillery or Command gives any ACC), then slap an ACC booster. Flares specifically have a 3 second initial cooldown and near permenent uptime, which gives them an edge over other ACC buffer/EVA debuffers.
Well, for now, nobody found any good use for Flares. Unless if you plan to pioneer this shit, don't bother with all the data you will likely waste.