The Great Cleric 12

Luciel and friends rush into the labyrinth after Sharzah, but when they finally catch up to him they're confronted by a mighty red dragon, as well. Backed into a corner and unable to retreat, Luciel makes a daring move and fends off the dragon with… Substance X, of course! Having proved himself in the face of not only corruption, but a dragon as well, Luciel is hailed as the city-state's dragon-slaying hero and asked to become their new leader. He agrees to a limited term of one year, but there's a lot to be done if he plans on making Yenice into a truly "free and equal" state…

Volume 12 of The Great Cleric manga series by Hiiro Akikaze.