Nanashi no Tensei Queen's Blade

The spinoff manga centers on Nanako Tadano, an office worker who died from overwork, and is reincarnated as Nanashi in another world similar to her favorite anime, Queen's Blade Legend. Nanashi accidentally frees a legendary monster and becomes a beautiful fighter with the most powerful skill. But in the world of beautiful fighters where everything is decided with harsh duels, Nanashi decides to hide her great power and just watch the great duel battles between her favorite characters.

(Source: Anime News Network)