Igougata Kyokuchi Geigeki Tenshi NANAKO ~Kaku Tatakaeri, Otome-tachi~

Humans have built a huge defensive city 3000 meters underground to save themselves from mysterious "vector" enemies. However, as resources get exhausted in constant battles, humankind throws last funds into new type of weapon IMA-01 Nanako. This weapon is given to main character as the last hope for survival.

[From vndbreview]