Valkyrie Bullet

In the near future, monsters known as giju have crossed over from another world and invaded Earth, dragging humanity into an endless fight for survival. Ordinary weapons do nothing against them, so the planet's only real defense is the Valkyrie Dresses, powered suits of battle armor with remarkable abilities that only women are able to pilot. That premise is upended when Ren Hifumi, a man, is sent to the Third Academy Valkyrie training facility. After surviving an experimental surgery with a success rate below ten percent, Ren serves as the test pilot for a new humanoid weapon meant to reshape how the war is fought, all while having to attend an all-girls school.