By Kuzuryu in the official forum

This guide is purely based on my own experience, so a lot of work/research still needs to be done. Your inputs are most welcomed here.

IGN: Kuzuryu
Village: Lightning
Server: Fiery Ridge (06)
LN (Las Noches) Milestone: Floor 103 at Lv 48

Introduction

About Las Noches:
Las Noches can be considered as one of the main sources of experience for high level players (other than medals for exp). It is one of  the two areas accessible inside the Demon City (click World Map on the top right hand corner of your screen, then click the top middle area on the map, it is a city with blue mountain as the background). Once inside, click the bottom middle area of your screen to interact with Kisuke Uehara, the person in charge of this place.

Choose to enter Las Noches (you only have 2 tries per day, so use them wisely), and he will give you another 3 options:

First option: pay 1000 stones to start from floor 1 (this is the basic starting place for everyone)
Second option: pay 20 gold to start from floor 51 (unlocked once you have reached floor 50 at least once, grant you roughly 40000 exp to begin with)
Third option: pay 50 gold to start from floor 101 (unlocked once you have reached floor 100 at least once, grant you roughly 120000 exp to begin with)

Once you have chosen an option, your Las Noches journey will begin. Another box will pop up with a few options:

Start Challenge/Continue Challenge: to proceed to the next floor, as simple as that.
Auto-Challenge: pay 28 gold to automate your Las Noches journey once, or 58 gold to automate your Las Noches journey twice (you will be ejected out of Las Noches once you die and all experience will be automatically claimed).
Buy Faux Body: to purchase 1 Faux body for the price of 28 gold (you can only purchase 1 Faux body per Las Noches tries).
Claim Rewards: to claim the accumulated experience so far (use this if you have no more Faux body left, as losing to a mob without any Faux body means 50% deduction from your accumulated experience, and you will be ejected from Las Noches immediately).
Leave For Now: to leave Las Noches without losing your current progress. You can go back to town to replenish your health/chakra or restock on potions.

Uehara will also give players quests to kill bosses on floor 10, 30, 50, 70, and so on.