At 6 AM, on Day 13, when Hope warps Lightning back to the Ark, The Final Day in Lightning Returns: Final Fantasy XIII begins, a day when the time stops, and you will have to focus your attention on the final boss, Bhunivelze.
As you return to the Ark, you will notice that Hope is missing, being replaced by Mog. Talk to him, and return to the warp device that will take you back to Luxerion.
Note: Assuming that it is your first playthrough, at this point you should have only one unfinished side-quest, named Last One Standing.
After the cutscene showing Lumina and Lightning, you will find yourself in Cathedral Plaza. Look around and you should see that on the right side of the street there are several merchants.
Make all the necessary preparations, because you will enter a smaller dungeon which you have to explore before getting close to the final boss.
Next, climb the red ladder on the right side and you will find an Adventuring Essentials Merchant. Buy all notes he sells (Sugriva Notes, Wendigo Notes, Set Notes, Neela Notes, Chimera Notes and Arcangeli Notes.)
Now, go down and head towards the exclamation mark to trigger a new cutscene. Watch it and stay alert because you will be attacked by several Cathedral Sentries. A Cathedral Sentry is an accessible enemy, vulnerable to all spells, which means that they should not be a problem for Lightning.
Aerora should help you stagger them faster, and the physical attacks to remove them from the battlefield. When all of them are defeated, enter the Hall Of Devotion and cross it, moving towards the next exclamation mark, where you should see a Chimera. Fortunately, Noel steps in, and after a new cutscene you will be sent to the Temple Of Light.
Note: The Temple of Light is a linear dungeon, inside which you can’t get lost, because there is only one tunnel that will eventually lead you to the next area. However, while exploring it you will find three adjacent chambers, each of them containing a sphere.
Open the said spheres to get the following items: Warrior’s Plume, Holy Revelations and Samurai’s Comb. As you follow the main passage don’t run away from your enemies, because in this area you will be attacked by Sugriva, Arcangeli, and later Chimera.
Note: Kill as many as you can, because you have to make the Omega Sugriva, Omega Arcangeli and Omega Chimera spawn for the Last One Standing quest.
Continue to advance through the dungeon until you find Fang. Help her defeat the Chimera using ice-based attacks and physical attacks, because you will heal the beast if you cast lightning or wind.
Now, that Fang joined the party, open the next door to find Vanille and start a longer cutscene, interrupted by a fight. This time you will have to defeat several Set. A Set is practically a more powerful version of the Anubys, easy to stagger by guarding the physical attacks. Once they have been defeated, the cutscene continues and Snow shows up just in time to stop the guards.
At the end of the cutscene all of Lightning’s friends will leave and only Mog will remain, acting as a merchant. He will sell you recovery items and also allow you to boost and synthesize your abilities. Once you talk to him you will have two options. The first one is to open the door and enter the Chamber of Creation, while the second one is to complete four trials.
Recommended is to complete all trials because you will get several rewards that will help you beat Bhunivelze. Practically, you will need to open all 4 doors around the one leading to Chamber of Creation and get to the other side of the passages.
At the end of each passage you will find a sphere and you will open a door allowing you to return to God’s Sanctum.
If you complete all trials, you will be able to retrieve the Ultima Weapon and Ultima Shield, while for opening all spheres you will receive four new level 1 spells: Chill, Flare, Surge and Tornado.
Note: Completing all trials is essential if you want to finish the Last One Standing quest, because you will encounter more enemies that will help you find the final three Omegas.
If you successfully completed the Ultimate Lair, and eradicated all species, during these trials you will only fight Chimaera, Sugriva, and Arcangeli.
Once you defeat their Omegas, talk to Mog and replenish all items to start the final battle. For this battle you should have at least the following abilities equipped: Deprotect, Deshell, and Slow or Imperil for debuffing the boss. Aerora is also the most valuable spell because it will allow you to stagger Bhunivelze. Once staggered, you will need Heavy Slash to deal physical damage.
Mediguard or Heavy Guard will be required to block Bhunivelze’s attacks. With these abilities and the Ultima Set equipped, open the door and head towards the final boss. Note: The boss fight features four phases, during which you should use the following tactics.
The first phase is timed, meaning that you have to finish it before the counter above Lightning’s head reaches 0. Start by casting Deshell, Deprotect and Imperil or Slow, to debuff Bhunivelze. Once debuffed, cast Aerora to stagger the boss and then Heavy Slash to empty the health bar.
Don’t forget to guard as many attacks as you can, and save your EP points for the final phase. Don’t end the first phase without casting another series of debuffs on Bhunivelze, because they will transfer to the next phase.
When the second phase starts, Bhunivelze should be debuffed, and you will continue to cast Aerora to stagger your opponent. Bhunivelze will try to hit Lightning using magic attacks, and most of them can be guarded; however, at some point the boss will spawn several balls known as Monads.
The spell itself is called Monad Beget. When you see the Monads, you have to take care of them, or Bhunivelze will cast Wings of Destruction, ordering all active Monads to self-destroy close to you. With the Monads removed, continue to use the same tactics as those you have used in Phase 1.
Again, don’t consume your EP Points, and cast all debuffs before starting Phase 3.
The third phase is focused on guarding, which means that you need to perfectly time your guards. Since Bhunivelze is already debuffed, Aerora remains the best option for you to stagger your opponent, as well as physical attacks to deal damage.
Switch from one schemata to another when you see Bhunivelze trying to attack you, and guard as many attacks as you can, because they can be devastating. You can complete the third phase without consuming your EP Points.
At the end of the phase, don’t debuff Bhunivelze because when the last phase begins, all debuffs will be removed.
The fourth and final phase is also the most difficult, because Bhunivelze can cast Heartless Angel, summoning two dancing angels. When they stop dancing, they will attack you and instantly drain Lightning’s entire health bar. The attack however, can be guarded.
If you saved the EP points, now is the time to use them. Activate Overclock, followed by Aerora to stagger the boss, and then continue to use wind, fire, or ice attacks. When Bhunivelze recovers after the first stagger, activate
Overclock again and repeat the first step. Once staggered the second time, cast all debuffs on Bhunivelze, activate Overclock, followed by a series of Physical attacks to finish the battle.