Hajimete No Aku (M/F F/F TF MC)

"villainess" Yuki Shinonome
Victims: everyone that was at the school that day
Method: transformation gun
Effect: Original fun loving cosplay girl, Jiro Akeno used the gun he designed to give her the outfit of one of the executives of the evil organisation he was a part of, however things go ary when Shinonome's mind is affected because she was an ordinary person and begins to act like a member of the evil organisation. She steals the gun and after using it to turn nearly all the people in her class into her subordinates she uploads various cosplay outfits into the gun and starts using it even more. by the end she is returned to normal but kept the gun. In a later chapter main character Kyoko Watari puts on that outfit but is not effected like Shinonome was.

http://www.mangareader.net/352-24763-1/hajimete-no-aku/chapter-17.html

I so do love the light nature of this, and the idea of a evil cosplay Queen lol. I even suggested it for one part in Rico's new game.

But yeah, thanks for finding and posting this dood =)