To get you need to create a lvl 1 Monk, have 6 archers, Ash knowing Big Bang.

First, you need to set your dugeoun up.

Talk to your Monk and make one that has the following.

Level: 5-6 (or 19-20 if you putting on "Faliure" title.)
Floors: 84+
Area: Small
Enemies: Few
Ground: Normal
Main Foes: Slime or Feeble (Low HP)

Next, asign the title "Great!" if the lvls are 5-6 and floors 79-83 or,
"Faliure" if you have lvls 19-20 and floors 99.

Now, save you wouldn't want to go through all this again!
Enter the dugeoun, there should only be one enemy per floor and some floors will ahve no one on them.

Make sure you have 40 Slots open to put items in cause you'll need those open slots.

Skipping that...On the final floor you should be lvl 600+ (If you put on "Great!") Or all 150 if you put on faliure.

Now there should be 2 enemies on the final floor, The boss and a side kick.

kill the boss and confine a high lvl merchant or ash on to the divine sowrd the boss was holding. (It looks like yoshisuna.) if you merchant is 500+ you'll have a 50% chance to get in so bring 3 or 4.

The sword has weak stats at first but they grow with every kill you make.

P.S. DO NOT kill the sidekick if you have not gotten the DIVINE SWORD!