TL;DR: Edge of Tomorrow Flowerpot
Basically the "Omega" aliens mess with regular magic so it's not very effective, except they can't get innate magic totally suppressed. The world has united, statutes of secrecy are down, and the humans finally gets a victory somewhere (at, idk, Normandy or somewhere). So the military leaders gather an invasion force to press on. They bring in Harry, who's victory over Voldemort, the last Dark Lord, brought him fame and made him hugely popular with the troops, inspiring their loyalty. But Harry's got very little combat experience, and he feels like he's paid his due to society (he's also a bit traumatized from the whole Voldemort thing and a bit selfish). So he tells, idfk, Kingsley or the president of the world or somebody, to get fucked, he's not going on no suicide mission. So they arrest him and toss him there anyway.
Battle happens and it's a bloodbath. Harry quickly gets super sus cause everything he's seen shows that the aliens are usually better coordinated, but they seem to be purposefully allowing their numbers to thin to give the human forces hope. He's running around, dodging aliens, until he comes upon a giant blue one. It stabs him in the stomach. With a weary sigh he takes the knife out of his belt, his wand long since broken, and jumps at the beast, tearing through it's thin underbelly and being drenched in it's blood/whatever the hell it is. His body starts to dissolve and he thinks this is the end....
...then he wakes up on the same pallet of muggle equipment he did 24 hours ago.
And it happens again. And again. And again. Each time he learns more, lasts a bit longer, but eventually he dies, only to live again. He sees his friends, old friends and new ones, those he's only known for 24 hours, keep dying. So he gives up hope and wanders away to drink until he passes out, deciding he's done enough fighting.
But just as he puts the bottle to his lips he hears two people talking about a solider. The so-called "Spirit of Fire." The one who's going to lead them to victory because she can conjure fire that hurts the demons. She killed one of the big blue ones, they say, and that catches Harry's attention. So he sneaks into the officers training facility to try and tell this crazy story to the one person who, by some miracle, might know what the fuck is happening.
He gets past all the guards and comes upon her training. He calls out to her, and she looks up...
