Aria the Scarlet Ammo(2011)

Hidan no Aria

Aria is a volatile girl who’s top dog in assault at a school for young mercenaries. When she discovers her classmate Kinji’s an ace, she strong-arms him into being her partner, but she isn’t the only femme fatale who wants a taste of his combat skills. His secret? He’s only a hotshot when he’s turned on! Aria gets Kinji for one mission, but he’ll end up with more than one finger on his trigger!

(Source: Crunchyroll)