Ore dake Level ga Agaru Sekai de Akutoku Ryoushu ni Natteita

Ryuuichi Hasegawa, the top player in a strategic medieval RPG, is granted a special "bonus" by the developer and reincarnated into the game world—not as the protagonist, but as the villainous lord Erhin Eintorian who dies in the prologue! This means Erhin's survival is a race against time, for his domain is set to be invaded by a hostile foreign power in just one day. In order to save himself, Erhin must use the leveling system that only he can access and his knowledge of the game to prepare his forces. The only hitch, of course, is that there's no way a villainous lord left a functioning military at his disposal! Can Erhin forestall the tragic fate scripted for him, or is he doomed to play out his own demise?

(Source: J-Novel Club)