Hunsu Duneun Cheonmanim

In a world where super-powered heroes fight extradimensional monsters, Hyunsoo is just a puny supporter without any powers. That is, until a god-like grandmaster named Chunma from the medieval past shows up out of nowhere and becomes Hyunsoo's teacher. Visible to no one else, Chunma secretly tutors Hyunsoo in the way of the fist. But when his new powers force him to join the battle against the monsters, Hyunsoo is thrust into the front seat of a wild adventure, with Chunma along for the ride!

(Source: Tapas)