The World's Fastest Level Up (Manga) Vol. 2 (manga)

Using his unique skill to bypass the standard wait time between taking down dungeons, adventurer Rin has been able to level up faster than anyone else. Now he’s run into an unexpected problem, and inadvertently triggered a secret extra boss fight! His opponent is stronger than any he’s fought before, forcing Rin to fight tooth and bloody nail. Can he claim victory, or will this overpowered boss claim its first victim?