Alright that's good, but you should still save before going down there. The monsters can still gang up on you like crazy and can inflict bad statuses as well. The Subterra is, in my opinion, the hardest dungeon in FFXII and will give you a great thrill while navigating through it.

Stuff you should know:

Your map is disabled and certain chambers are dark. Meaning, you have no map, and no view of your surroundings, and five or six monsters can attack you by surprise.

Certain treasure chests down there can contain great items. You should save just in case these treasure chests don't spawn, or if you end up only getting 1 Gil or Knot of Rust when you could have gotten Ribbon or Ultima Blade.

And whenever you see a Magic Pot, TURN OFF ALL YOUR GAMBITS. You do not want to attack it because it most of your attacks will do 0 damage (excluding 1000 Needles) and it will hit you for about 4000 Damage with its physical attacks, and can cast powerful magick like Flare, which can do over 7000 damage. Give it an Elixer like it asks, then it will be happy and will run around in glee. During this, you can steal back your Elixer that you gave it and kill it without it fighting back.