Tales of Vesperia: Definitive Edition

I'm going to once again assume you've been following kouli's side-quest guide. The "Extra Dungeon" side-quest labels in his guide are what you need to open this dungeon, which will finally be accessible after beating the main story. I'd leave this until your speed run/max level playthrough. Note that you also have to complete the Giganto monster quest to have all the routes available.

As for the dungeon itself, it's really not much of an issue. You will face off against enemies and bosses you've killed before and find chests to open. You will periodically get "Fake Gald" items from these chests or from the bosses, and you need 15 of them to face the final boss of the dungeon. Keep repeating runs of the dungeon until you have enough (you need four runs minimum to scan all the enemies, but it would take many more to get to Level 200 and 10 Million gald), then tackle the Traitor to Heaven.

You can use this map to navigate the dungeon. There are some extra bosses added in this version but just take all four main routes to fight them all.