This is effectively a replay of the final sequence of the game, but with one extraordinary surprise -- the final boss is none other than Shin Amon, who previously appeared in Judgment. There is a very substantial storyline tick here that hints at an answer to "the" question everyone had at the end of Yakuza 6.
Generally speaking, you want to be at level 65 with all six of your characters, with level 55+ in at least one job, and you want a variety of high-powered essence attacks covering a variety of attack types. I suggest having 900 HP minimum on all characters.
All through the dungeon, you will come across challenging variations of monsters you've seen throughout the game, but no brand new monsters. The gameplan should be constant essence moves that attack every opponent. Ichiban's Essense of Orbital Strike and Zhao's delightful ladder attack should be done every turn if possible.
Along the way, you will also fight reincarnations of Saejima, Majima and Kiryu. None of them are quite as difficult as in the main story line.
Shin Amon will be coming at you with a smorgasbord of different attacks; everything from his own Essence of Orbital Strike, to (probably unavoidable) attacks that will take out one of your characters in one shot. He is nowhere near as difficult as Amon fights in previous Yakuza games, owing largely to the fact that you are in control of the pace. But you better come equipped with lots of party healing items, and you better have upgraded your weapons at least 12 times each.
One last thing -- while you will get the trophy for finishing it once, you will absolutely want to do this dungeon multiple times, as the story portion final sequence actually changes. Plus if you've got it in mind to go after the "Victory of the Millennium" trophy, you're likely going to be spending many hours in this dungeon, so pay close attention to what attacks work best on what monsters and tune your party usage accordingly.