Things I remember:
It was a manhwa, 100% Korean. Not sure if it was an isekai. The first level consisted of a dark passageway/tunnel, with different colored arrows/darts (not sure which) being shot at the MC. He discovered that every time he interacted with the projectiles (deflecting, catching, etc.), he would increase his stats.
IIRC the arrows/darts were different colors, each with different correlations. Red = ATK(?) Blue = speed? mana?? (not sure) Yellow = HP
The MC ended up repeating this first level so many times he was kind of OP by the time he actually left. I think he repeated the first level by buying special passes from the shop that allowed you to repeat the level, but were widely considered useless items as most people don't want to repeat levels and just move on. I can't remember what happens after this part to be honest... does anyone know of a manhwa that fits this description? Thanks.