This is part 41 of my SAO summary / fanfiction, and you can start from the beginning here.
Yukki beat a few more bosses since she hadn’t died yet, and she loved swordplay so much. With Kirito in her party, it was too easy though.
She fought a duel with Kirito to see who was the best at swordplay. Kirito would have won but time ran up. He let her win, since she was a girl. And if he’d gone any further in swordplay with her, he would have gotten AIDs. Asuna wouldn’t have liked that.
However, one day Yuuki had a heart attack. It turned out she’d been ganbarimasuing so hard she cured AIDs. But she still had to die, so that Kirito could unambiguously be the best at swordplay again.
Yukki died. Asuna asked her to spend her last moments inside a video game so she wouldn’t have to look at her ugly face in real life. With her dying breath, Yuuki passed on one of her swordplay skills to Asuna. Lots of faerie people came to watch Yuuki die in the hopes she would give them a sword skill too. She didn’t.
Yuuki died. Her last moments were the happiest moments of her life.
Everyone else lived happily ever after. They didn’t learn from Yuuki’s death and continued practicing unprotected swordplay.