From Horobi no Michi:

Kirisaki Fumio's just a highschool girl in love...

...but then she finds out that the guy she's fallen for happens to love someone else.

...so she decides to go out and get a chainsaw.

And take it to school.

And then many things happen.

The story revolves around Koyomi Morishita, a clumsy high school freshman girl who is often mistaken for a grade-schooler because of her shortness. Koyomi becomes a disciple of Misa Anehara, a 25-year-old graduate student who happens to be one of the most powerful modern-day magicians.

From MangaHelpers:
The world is in a war against an alien race called "Mimics," who have taken over most of the world with a mission to eliminate the human race. Keiji Kiriya is a new recruit in the United Defense Force, which is battling against these "Mimics." On his first day of deployment, Keiji and his unit encounter "Mimics" and are all killed... Or was it all a dream? For some inexplicable reason, after every time he dies, he is resurrected and returned back to the day before the battle. Why does this happen to Keiji and how can he escape from this never ending cycle?

Note: Nominated for the Eisner award for the "Best U.S. Edition of International Material - Asia" in 2015.