Omega Mark XII is the most difficult boss battle that can be fought during a regular playthrough of the game. The only fight that is arguably more difficult than Omega is the battle against the Judges in Trial 100 of the Trial Mode feature.
Omega can be found in the Great Crystal. The guide below will assume that you have already traveled through to the top of the Great Crystal to obtain the Esper, Ultima. Before you set out for Giruvegan to make your way into the Great Crystal, make sure that you have the following:
Reverse
Renew
Hastega
Bubble Belts (x3)It helps to be fairly high level but there are strategies included below that can make this battle easy even for a lower leveled party.
The following instructions provide a step-by-step walkthrough of how to reach the platform where Omega Mark II resides. An alternative approach, if you are comfortable with reading the maps, would be to head over to the “Treasures in the Great Crystal” page and use the maps and guide to access the area. Otherwise, follow these steps:
There are two pathways leading up – take the pathway on the left.
Examine Way Stone IX and choose to “touch the device”. This will warp you to Way Stone X.
There is only one pathway leading off the platform. Take the pathway upward to the next platform.
Approach Way Stone XX and choose to “touch the device”. This will warp you to Way Stone XIX at the top of the crystal.
Take the only pathway leading off the platform going downward.
There is a Save Crystal in the center of the platform. You should definitely consider saving your game before attempting the battle against Omega Mark XII. There are two pathways leading off this platform – the one you arrived on (on the left) and the one on the right. Take the one on the right.
Approach Way Stone XVIII and choose to “touch the device”. This will warp you to Way Stone XVII.
Take the only pathway off the platform leading downward.
Take the pathway on the right, beyond the Giruveganus, leading upward. Note that if you run into a Virgo Gate along this pathway you can also take the long way around to reach the next platform as shown in the map below.
Take the pathway on the opposite side of the platform leading upward. You may run into a Libra Gate along this pathway – if so, you can head back to Sthaana Libra and activate the Gate Switch and open the Libra Gate. Use the map below to help you navigate to the Libra Gate.
Take the pathway directly across from the side that you entered, traveling past the Ash Wyrm, heading upward to the next platform.
Travel up to the top of the platform and activate the Capricorn Gate Switch. Head back down to the previous platform.
Go straight across the platform again (past the Ash Wyrm) and head down to the next platform.
There are two pathways on the opposite side of the platform – take the pathway on the right leading downward.
There is a Libra Gate Switch on this platform but don’t activate it. Take the pathway on the left leading downward.
Ignore the Gate and run past the Skulwyrm down the open pathway leading downward.
Touch the Capricorn Gate to open it and then turn around and head back up.
Take the pathway on the left, past the Skulwyrm again, leading upward to the next platform.
Run past the Libra Gate Switch and take the platform on the right leading upward.
Travel to the top of the platform and take the pathway on the right leading downward. If there is a Virgo Gate blocking your path, take the long way around as shown in the map previously and show in the map below:
Run past the Giruveganus and take the pathway on the left leading upward.
Head up to the top of the platform and examine Way Stone XVII. Choose to “touch the device” and it will transport you to Way Stone XVIII.
Take the only pathway off the platform leading downward.
There is a Save Crystal on this platform which you can use again. Take the pathway on the left leading downward when you are ready.
Head down to the platform and examine Aquarius Gate Stone and “touch the device” to activate the timer. Head back up to the previous platform.
Take the pathway on the right side of the Save Crystal.
Head up to the top of the platform and examine Way Stone XII and “touch the device”. It will transport to you Way Stone XVII.
Take the pathway on the right past the Giruveganus (unless there is a Virgo Gate, in which case you will need to use the map below to go around) leading upward.
Head up to the platform and past the Skulwyrm and up the walkway on the other side.
Head past the Ash Wyrm and take the pathway on the right leading down – touch the Aquarius Gate to remove it and continue along the pathway.
Take the only other pathway leading downward.
Examine the Taurus Gate Stone and “touch the device” to activate the timer. Take the pathway back up to the previous platform.
Head up to the platform and take the platform on the opposite side leading upward.
Run past the Ash Wyrm and take the pathway on the left heading down.
Run past the Skulwyrm and take the pathway on the right heading down again.
Ignore the Libra Switch and take the pathway on the left leading downward.
Run past the Skulwyrm taking the pathway on the right leading downward.
Take the pathway on the opposite side of the platform, past the Ash Wyrm, leading downward.
Remove the Taurus Gate and take the other pathway leading downward.
Approach Way Stone XXI and choose to “touch the device”. This will transport you to Way Stone XXII.
Take the only pathway down away from this platform into the next area.
Approach Way Stone XXIII and choose to “touch the device”. This will transport you to Way Stone XXIV.
Omega Mark XII is located on the next platform (Dhebon Jilaam Avaa).
There are now two strategies that you can use to defeat Omega Mark XII. The first strategy involves using the Reverse spell to cause Omega’s damaging attacks to heal your party members.
Omega only has one attack that it will use but that attack is extremely strong. This strategy nullifies the strength of Omega’s attacks and can allow even a low-leveled party to defeat the boss.
The downside to this strategy is that there is a random element to whether it will be successful. Omega’s strong attacks also have a chance to inflict the Berserk status effect which can cause a chain reaction and wipe your entire party. There are also brief instances while Reverse is being recast where a party member can be KO’d.
The second strategy is essentially a brute force attack. It involves withstanding a few of Omega’s heavy attacks while one of your party member applies the 
Wither Technick (as well as the other break Technicks afterwards). Each of these strategies is detailed further below.
Level: 99
HP: 1,037,069
Steal: 
Knot of Rust (3%)
Reverse Strategy
Set up one of your characters with some Gambits to automatically cast Reverse on each of your party members. Ideally though, you want this party member to cast Reverse on themselves before casting it on other party members. The Gambits should be:
Self > Reverse
Ally: any > Reverse
Ally: any > Arise
Ally: status = Berserk > Dispel (or 
Dispel Mote)
You may also want some other helpful Gambits such as:
Self: MP < 10% > 
Hi-Ether
Ally: Vaan > Hastega
You should also consider setting up your other characters with a Dispel or Dispel Mote Gambit as well in case your main ‘Reverse’ party member is inflicted with the Berserk status. There are no other party to this strategy other than to slowly take Omega Mark XII now that its damage is completely nullified.
Brute Force Strategy
This strategy involves surviving a few of Omega’s attacks while one of your party members uses Wither to reduce Omega’s strength. Eventually, you can get its attacks down to less than 1,000 damage which becomes much easier to manage.
Prepare all three of your party members for the initial onslaught, including preparing their gear, the initial spells and their Gambits. Let’s start with gear:
Equip all three of your party members with a Bubble Belt. Casting Bubble Magick is a much less effective route of increasing each of their HP because it will need to be recast each time that party member is KO’d, which will potentially be quite a few times using this strategy.
You should also equip all of their strongest armor and equip all three party members with a Shield if possible. You should focus on surviving the first portion of the battle rather than trying to do damage.
Set up Gambits so that 1 (or more) of your party members are casting Renew or Curaja as often as possible. Setting up a Ally: HP < 70% > Renew is ideal. Having two party members that can cast healing spells, while the third focuses on using Wither, will increase the odds of your party surviving. Make sure that they have an Arise Gambit or a 
Phoenix Down Gambit set up as well.
One party member should continue to use Wither until Omega Mark XII’s attacks become manageable. Once you reach this stage, switch out your party member’s equipment for equipment that has higher attack damage (removing the Shields previously mentioned above) and reduce threshold used for your party members casting healing spells so that they can all start damaging the boss.
A video of the battle in the Zodiac Age version is included below:
The rewards for defeating Omega Mark XII (the loot dropped) includes the Omega Badge. This badge is one of the ingredients used to create the 
Wyrmhero Blade, the most powerful sword in the game.
This is also one of the enemies that most players leave until last but is required to complete the Bestiary and obtain the Scrivener Trophy.
This side quest battle is one of the last portions of the end game content available in Final Fantasy XII. Click on the following link to return to the end game side quests page.
Alternatively, you may wish to switch over to completing the Trial Modes available in the game, as some of these battles are even more difficult than the battle against Omega.
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