FFXII - Walkthrough - Dreadnought Leviathan
Your Location Map clearly indicates where to go (circled red x) so make your way there. Heed Vossler's warning; tripping the alarm will cause the arrival of a few soldiers to fight, but there's not much difficulty in eliminating them.
Boss fight in Central Brig Access: 2 Judges, 3 Imperial Soldiers and an Imperial Magus. Take out the judges first; the rest will follow. The Magus casts Blind, so have your Eye Drops ready.
Once it's done, you'll get the "No. 1 Brig Key". Use the Brig No. 1 Key on the door, and enter.
Four rooms here; the first two have a moogle selling equipment (top of the line stuff, so get some upgrades if your weapons are inferior) while other has the "Systems Access Key" (to be used later) and a save point. Save your game. Now check Cell C-202 for Ashe; she joins your party!
Head North to the Sub-control Room. While it has many soldiers, it also has the Security Terminal. Using the "Systems Access Key", you can disable the alarm for a period of time, which is nice. So, go up the stairs, and at the "Security Station", use the key.
Observe the minimap and Location Maps carefully as you plot your escape; the hallways are riddled with dead-ends, so you need to plot out the most efficient path to get through it all.
At the Northeast square area of "Port Section", another cutscene; Vossler leaves the party and Penelo joins it. Continue to the circled Red X.
Judge Ghis
Steal: Potion
Attacks: Protect, Lunge (~70), Kick (~70), Aero (~170 all), Greater Barrier (Shell)
Pretty straightforward other than Aero, which will be very painful if too many people are too close to him, so try to keep some people back. If you have some Quickenings, feel free to rattle off a combo once you've take off about half his HP.
Cutscenes. When you're back in Bhujerba, do what you feel like doing: head to Ondore's place when you're ready. More cutscenes, and your new missions; venture across the massive expanse known as the Sandsea to get to the Tomb of Raithwall.
The Sandsea is split into two: the Ogir-Yensa Sandsea, and the Nam-Yensa Sandsea.