This chapter of the game begins with Sazh and Vanille arriving in the fantastic city of Nautilus. It’s very similar to Gold Saucer in Final Fantasy VII, Esthar in Final Fantasy VIII and Luca in Final Fantasy X.
You have a whole bunch of Crystogen Points (CP) that can be spent in both Sazh’s and Vanille’s Crystariums, but before spending them, consider the helpful hint noted below:
In the chapters leading up to this point, it has been easy to fully complete each character’s available Crystariums and completely (or nearly completely) fill out some or all of their roles. At this point though, and moving forward, that is going to be much more difficult to do.
From now on it is recommended that you be strategic about how you spend your Crystogen Points. The most important thing to remember is that you should spend each character’s CP to get to specific abilities on the Crystarium first rather than deviating off the main pathways and trying to fill out the Crystarium. This will allow each character to get important abilities as quickly as possible before focusing on minor stat increases. Keep that in mind as you follow the steps outlined below.
There are a couple of important abilities that you should target in each character’s available Crystariums.
Starting with Vanille, make sure that you obtain her 
Dispel ability in the 
Saboteur Crystarium and pick up the 
Blizzard spell from her 
Ravager role. She can also gain access to 
Renew and Raise which can prove helpful for upcoming fights depending on how defensive you go with your play style. She will spend more and more of her time in her role as a Saboteur though so consider filling out this and the Ravager Crystarium first.
Sazh, on the other hand, will be operating as a jack-of-all-trades so there’s no definitive strategy to follow when spending his CP. For now, he will be your group’s only 
Commando, so there is value in working your way through that Crystarium, but there are a lot of beneficial spells and abilities in his 
Synergist tree which are worth obtaining. Two worth noting are 
Haste, which reduces the time it takes for each character’s ATB gauge to fill, and Boon, which extends the duration of positive status effects.
There is a 
Save Station nearby which you can use to save your game or purchase items. As a quick reminder though, you should minimize the spending of any Gil unless it is absolutely necessary, as fights in these earlier chapters of the game are not challenging enough to warrant needing to spend Gil or needing to replenish your inventory of items.
It has been reported by many Final Fantasy XIII players, on the PC version of the game specifically, that the game has a high likelihood of crashing in this area. The solution to this is to switch your game to windowed mode rather than playing in full screen while in Nautilus.
Your destination is marked on the mini-map. Move towards it for a short bit of dialogue between Vanille and Sazh as they watch the other team members on the television screens.
You will regain control of the two of them in the same spot and you will just need to move forward a bit further to kick off another cutscene as they exit this first starting area of Nautilus.
The map of this area is very straightforward. Just walk forward until you reach the next map marker along the Festival Road as there is nothing else to do in this first area.
Move forward towards the end of the pathway which has another Save Station up against the wall that you can use. The nearby map marker points out a transport that you can use to reach the next area of Nautilus. Press 
Button when you’re near the transport vehicle to “Board”.
Your first interactions with full-sized Chocobos in Final Fantasy XIII! There’s another short cutscene as the two enter. Turn around as soon as you can control your characters and follow the pathway on the left side, past the Chocobo, to find a Treasure Sphere that contains a tuft of 
Phoenix Down. Head down the hallway and into the next area when you are ready.
There is yet another short cutscene when you reach this area as Sazh’s Chocobo chick flies off. This is a short, easy mini-game called “Flight of the Chocobo” where you need to track down the Chocobo Chick in a few areas around the Chocobo Corral. It’s very easy to find it and it’s unlikely that you need help with the locations, but they are listed below just in case:
Button near the souvenir cart in the middle of the corral.
The herd of Chocobos will leave after you “examine” the Chocobo chick for the last time and a Treasure Sphere will float down from the sky. Open it to pick up a free 
Star Pendant. This accessory increases a character’s resistance to 
Poison so no need to equip it just yet.
Exit the Chocobo Corral area by going through the area where the flock of sheep had previously been standing. There is a Save Station you can use to save and, once you’re ready, move forward to the next map marker for a cutscene before you reach the next area. It’s an ambush!
Despite the guards seemingly running up to and engaging Sazh and Vanille in the cinematic, you’ll have a chance to revisit the menu screen and set your characters up one last time before running up to engage the PSICOM Soldiers yourself. Do any last upgrades, equipment swaps or CP spending that you need to and then kick off the first fight.
These enemies are difficult to ambush with a preemptive strike due to their numbers and their patrol patterns, so don’t waste too much time before engaging each of them. The first group includes six Corps Gunners which are enemies that you have already faced many times. There’s no need to use Libra as their 
Datalog Intel Entry is likely already complete, and they are extremely easy to defeat.
Have Sazh stay in his Commando role while Vanille deals damage as a Ravager ( Slash & Burn) only switching to War & Peace if either of your character’s HP gets too low. The next group of enemies after this first one includes an Orion which makes it a bit more difficult than the first. Focus on taking out the Corps Gunners first and then focus attacks on the Orion.
The next wave includes a group of two Zwerg Metrodroids which, despite looking like enemies that you have faced previously, are brand new. They are easy to defeat but make sure to use Libra on them to complete the Datalog Enemy Intel entries and reveal the droid’s weaknesses (fire and earth).
Walk up to the next ledge and defeat the solo Zwerg Metrodroid standing there. The Treasure Sphere next to it contains a pair of 
Spica Defenders which are a new weapon for Sazh.
This weapon provides high Magic and the “Augment Maintenance” ability which increases the duration of positive status effects (
Bravery, 
Faith, Haste, 
Vigilance, etc.). This is normally a great weapon for Sazh but because he will be spending most of his time in this chapter as a Commando you should leave his Procyons equipped.
The next group of enemies includes two Zwerg Metrodroids and three Corps Gunners. The next area of Nautilus, called “The Clock Tower”, begins halfway up the ramp.
The final “mandatory” group of enemies to fight, as you continue up the ramp, includes three Zwerg Metrodroids and an Orion. Take out the Metrodroids first and then focus on the Orion.
Behind this group is one more Orion enemy that you can defeat to gain access to a Treasure Sphere. This one is a bit easier to get a preemptive strike on because you can bait the Orion into charging at you and then attack it when it turns around to return to its previous spot.
The Treasure Sphere behind it contains a 
Guardian Amulet. While this accessory provides a fair bit of defensive protection, it's not worth equipping to either of your characters right now.
Follow the platform up towards the Save Station. There is another group of Zwerg Metrodroids just up ahead guarding the last Treasure Sphere in this area. Defeat the Metrodroids, which will probably require swapping to War & Peace for some healing given how many of them their area. Then grab the 
Healer’s Staff out of the Treasure Sphere .
This is a great weapon when Vanille is relegated to being a 
Medic which is not often and not right now.
Save your game before proceeding as there is a boss fight coming up. Make sure you’ve set up the Tide Turner paradigm (Sazh as Synergist and Vanille as Saboteur) and set this up as the default paradigm. Also, set up a Symbiosis paradigm (Sazh as Synergist and Vanille as Medic).
Start by using Libra on Midlight Reaper to identify its weaknesses and then have both Sazh and Vanille get to work applying their buffs and debuffs respectively. Sazh should apply Haste, Bravery, and Faith to both characters, and he can also apply 
Enfire to himself to exploit the boss’s vulnerability to ice and increase the damage that he does as a Commando.
Vanille will, ideally, be able to apply 
Imperil quickly, which will increase the damage Midlight Reaper takes. This is the only really important debuff for her to apply, so swap to Symbiosis so that she can keep the team healed up and use Dispel to remove the Poison status effects while Sazh continues applying all the buffs listed above.
Move into an all-out attack once Sazh’s buffs have been applied. Start with Slash & Burn and rotate into Dualcasting when you can to pump up the chain bonuses. This boss is easy to Stagger, but make sure that you make every effort to inflict as much damage as possible while it is Staggered and increasing the Stagger gauge as high as you can, because you will need more than one Stagger to defeat this boss. Ideally, though, you will only have to Stagger it twice.
Be prepared to move into War & Peace if the damage gets too heavy, especially after the first Stagger phase, and don’t forget to swap Vanille back to her Saboteur role when the Imperil debuff falls off the boss.
A message will flash above Midlight Reaper after the first Stagger that indicates: “Main Cannon Primed”. This means that an attack dealing heavy damage is coming, so make sure that your party members are healed up and ready to recover quickly. If you’re lucky you may be able to Stagger the boss a second time and defeat it before it uses this attack.
This boss will drop a key item called 
The Motherlode which will allow you to access “The Motherlode Retail Network”, a new shop accessible at Save Stations that sells weapon catalysts.
A lengthy set of cinematics will take place after the fight before you find yourself back in control of Sazh.
Vanille will no longer be in your party, but the boss coming up will deal heavy fire damage to your party, so consider equipping Sazh with an 
Ember Ring which will help to reduce the damage.
Walk forward and up the stairs towards the map marker to initiate the next battle and don’t worry, Vanille will be rejoining you, so no need to mess around with the paradigms.
This is your third Eidolon battle following Lightning’s battle against Odin and similar principles from that fight carry forward. 
Doom will be cast on Sazh at the beginning of the fight initiating a countdown timer. The fight is over if the timer reaches zero. This Eidolon battle tends to not be as challenging as the last one.
Brynhildr yields to those who amass chain bonuses and to those who strengthen their allies, according to the enemy intel report. Speaking of which, make sure to use Libra twice and early on in this fight. Because of how she yields and how her gestalt meter charges, you can follow a regular battle strategy to win this fight.
Like the previous boss, have Vanille apply debuffs while Sazh provides buffs (Haste, Vigilance, etc.). Switch over to Symbiosis to have Vanille heal up the party while Sazh continues buffing the party.
From there it’s just a matter of rotating between Slash & Burn (to keep the Chain Gauge bonus high), Dualcasting (to pump the Chain Gauge) and War & Peace if your party members need to be healed. Continue with this attack pattern until the gestalt gauge charges and then press the 
Button.
There are a few more cinematics that take place after the battle has concluded before the game moves on to the next chapter.
