Masou Gakuen HxH Jumpchain

From the introduction: Hail, Jumper, and welcome to Earth, again. What’s the flavor of crisis this time? Interdimensional invaders. Though for a while you will only ever see the mecha-weapons this enemy deploys wholesale. Of course, in true anime fashion, all the world’s proper military got crushed and humanity has fled the land to dwell aboard floating (and some submersible) mega-cities crammed with what cultural artifacts that could be saved. For the time being, Humanity hides, biding its time to strike back and reclaim the Earth.