From what I remember, the MC is a guy who was transported/isekaied/summoned with a group of others. The ‘FL’ who had always or recently been kind to him is being sent as a sacrifice to some dungeon or something like that, but he offers to go in her stead; turns out she’d done so under orders of another guy who was part of the Isekai group and has been banging the ‘FL’ (apparently the guy is pretty strong and she is selling herself to him to live?) but the rest of the group is pretty happy to toss MC in. MC is cast into the dungeon and swears revenge. He finds and fights a few monsters, the most notable that I can recall being the no-life king, and he takes some skills from the defeated NLK plus a cloak; iirc his build develops into something of a dodge dps w/handgun(s) and his blood is poison? It’s been completed, and a fair bit ago…
While not all of it fits, I'm getting a lot of Arifureta flashbacks from your post. You might as well check it, if you didn't
No, there wasn’t a harem, and Hajime wasn’t actively cast out into the dungeon alone by the whole class but rather fell when they were all fighting a behemoth (‘stray’ fireball), which didn’t happen to the MC from the manga I’m looking for. But I do see the other similarities…