Heavy Object

They fight a giant weapon with a gun.

Wikipedia
Heavy Object (ヘヴィーオブジェクト Hevī Obujekuto^?) is a Japanese light novel series, written by Kazuma Kamachi and illustrated by Ryō Nagi. ASCII Media Works have published eight volumes since 2009 under their Dengeki Bunko label. It has received three manga adaptations. A 24-episode anime television series adaptation by J.C.Staffaired from October 2, 2015 to March 25, 2016.