Lasboss x Hero

Sagara Masamune is a manga author who isn't very successful, and it's looking like it's time to end his battle manga. To go for an unexpected ending, he plans to kill off the main heroine, Nina, in a dramatic way. As he works on his manga, he is mysteriously sucked into the pages. Now he's trapped in the world of his own creation, with his protagonists, exorcists Haru and Nina. He doesn't really get what's happening, but he thinks concluding the manga, which involves orchestrating Nina's death, might get him back home.

(Source: MangaHelpers)