Solo dungeon raider

15 years ago, these things called "dungeons" and "towers" with high and low leveled monsters appearing out of nowhere and almost destroying half of humanity, but then these people named "hunters" saved humanity who was in a brink of destruction, most people from then on dreamed on becoming a hunter, but It's not as easy as it looks to become one, the protagonist, Lee Jun-woo became a hunter 15 years ago, but he never moved a rank nor had any skills, when he and his teammates would go to a dungeon, he would always barely make it out alive, he would always have life threatening situation when he goes to a dungeon, but he never gave up on his dreams on becoming a hunter, nevertheless he did his best but one day, he was in a life and death situation again, which is not new to him but this time... he actually died. but to his surprised he went back to the past! Additional Info: Towers are unbeatable and not easy to destroy nor demolish, and they also are the ones that sends these random "gates" everywhere in the world, these "gates" has monsters popping out of it and these monsters are the ones who are putting humanity on the brink of destruction, and "dungeons" has low and high level monsters, the rank goes from E-rank to S-rank, these "dungeons" help "hunters" to level up, but most "hunters" die because of the difficulty. and these "monsters" in dungeons or "gates" can be made into high quality materials, people use their skin, tooth or the stuff the "monsters" leave after dying, most people sell those items after they get one while raiding a dungeon, but some just keep them.