Animeleague

The original novel by Sakurazaka is set in a future Earth under attack by alien invaders known as Mimics. Humanity's infantry goes into battle in powered armor suits that enhance their deadliness, but they are still losing the war. Keiji Kiriya is a newly enlisted soldier, but dies in his first battle. After he dies, he finds himself awakening to a time prior to the battle, having inexplicably entered a time loop. Through a repeating process of dying and learning from his mistakes, he slowly finds a way to live longer and longer, and finds himself drawn to an elite soldier named Rita Vrataski.

Reminds me of the movie Edge of Tomorrow.