Death March to the Parallel World Rhapsody.

29-year-old programmer Suzuki Ichirou finds himself transported into a fantasy RPG. Within the game, he's a 15-year-old named Satou. At first he thinks he's dreaming, but his experiences seem very real. Due to a powerful ability he possesses with limited use, he ends up wiping out an army of lizard men and becomes a high leveled adventurer. Satou decides to hide his level, and plans to live peacefully and meet new people. However, developments in the game's story, such as the return of a demon king, may cause a nuisance to Satou's plans.