Haruka ends up at the bottom of a dungeon due to him goofing off, and has to solo (sorta) his way out. This is the same dungeon the otas and class has been dipping into, very briefly, as it's one of the unconquered impossible dungeons. At the same time he's soloing his way out, the class president is leading the class in a rescue mission, and it's very slow going because it's a stupidly hard dungeon. He ends up on floor 100, and they spend a chapter on 1-3 floors at a time as he slowly cheats his way up.
Also, the identity of the lady in platemail is hilarious.
The way he gets through the floors is really funny, though. He really cheats the hell out of things, like digging holes from the floor underneath the floor he's going to and piping in poison gas to kill a swarm of giant bees, or using cooking oil and fire magic to set fire to an entire dungeon floor full of treants, or digging holes under a bunch of heavy golems and letting them fall hundreds of feet to their deaths.