This is Skill Master Levels Up.
In a world teetering on destruction, humanity's only hope is becoming stronger in a mysterious tower offering four modes: easy, normal, hard, and the brutal "alone mode." Kang Taesan, despite choosing the easy route, becomes the most skilled player upon conquering the tower. But as doomsday nears, he realizes that he isn't strong enough to save Earth and turns back time, embracing "alone mode" to level up even more. With a second chance, can he amass enough power to prevent the end of the world?
Monsters have emerged from a crack in the sky in the modern world, and people were given the choice to enter a labyrinth on either easy, normal, hard or alone mode. While in the labyrinth, you can acquire various skills and level up your stats. Once you clear the labyrinth, you return to Earth with your skills and stats intact, but can't increase them any further. Also, the Earth you return to is now a post-apocalyptic wasteland swarming with monsters.
The main character -- a young man named Kang Taesan -- chose to clear the labyrinth on the easy mode, due to having low self-esteem. As a result, his stats are much lower than those of players who cleared it on hard mode. Despite this, he's more formidable than even the strongest player in the world -- a young woman named Lee Taeyeon -- due to having acquired more skills than anyone else (over two hundred in all). However, his relatively low stats prevent him from using over half of his skills, meaning that he would've been even more formidable had he cleared the labyrinth on a harder difficulty setting.
Because of this, the monsters are on the verge of wiping out humanity, despite the best efforts of the returned players to fight them off. Most of the players have already died during three previous waves of monster attacks, and the fourth wave proves too much for even Taesan and Taeyeon. As he's moments away from being killed by a powerful monster, Taesan activates a magic stone that he finds in the hand of Taeyeon's corpse. The stone reverses time to the point when the monsters first invaded Earth, and Taesan chooses to enter the labyrinth on the hardest difficulty setting this time: alone mode.
You can read the full manhwa at Tapas.