Okay. So I beat the robot. It's not that common in my experience to take on (what turned out to be) a 4-hour challenge that is only one continuous battle, so I thought I'd share what happened! I don't hold this up as a good way to do the boss, but I thought some people might enjoy reading through and you might even get an idea or two. At the very least I will get an idea or two from your responses, or so is my hope in composing this. I like to write in detail so if a lengthy, nuanced post of a struggle through the longest fight in the game is not your cup of tea I’ll understand if you stop reading here!

The team

I had listened to guide4gamer's points on delaying your challenge of Aigaion until we get some better units--but I was lucky enough to pull both Rem and Aileen from just regular daily summons, and I figured if I have the right personnel, then I should at least be trying it now. After a few skirmishes with the boss I knew this was a Moby Dick moment—he had to go down, whatever it took. I’ve never had so much fun with the game and I was definitely not about to wait until powercreep made it any easier.

My first attempt was with: Refia, WoL, Rem, Rikku, my Aileen, and a friend Aileen with 850 ATK. I have no source of Auto-Refresh and not enough crystals for Refia's Kazus Pendant Awakening, so I just kind of hoped Refreshing Tool would do the trick... many turns into the battle I realized not all MP restore moves are created equal. I killed the Left Arm once, then realized I was going to have too many mana problems to try to go 10 rounds, so I thought I'd try the Right Arm to practice chaining on it. I did notice Aigaion starting to attack more after its Right Arm was killed once, so I figured that'd be the last time I tried that.

Not having a better mana battery (no Ace, no Bran, no Lunera, no Tilith), I had to get creative about where the mana was going to come from. The damage and survivability was okay, but the longevity was not. I also figured I'd swap Y'Shtola for Refia because ever since WoL moved in I don't have any spirit buffs. The no magic achievement was right out the window from the get-go, lacking Tilith.

The team I settled on:

• Rikku
• Warrior of Light
• Rem
• Y'Shtola
• Aileen
• Friend Aileen

Step 2: The Setup (names link to unit calculator, amazing tool btw!)

Warrior of Light

I picked the Mighty Hammer, dimly aware that this could cause a mistaken lightning attack to kill my game... more on that later. I gave him Rikku's Pouch, figuring he would be the most likely person to have a free turn for Eccentrick. Besides that he was geared for HP, SPR, and used the Carbuncle Esper--I figured it was the best defensive esper (lots of SPR, my Titan had a more important place to be, and my Golem is not 40) and the chances that Y'Shtola would need the dispel were pretty slim. I thought for mana that he would be okay given that he takes a magic attack every turn and has his 25% chance to restore 25% of his mana--this works out to 6.25% autorefresh on average, and I had a few tricks up my sleeve if there was a long dry spell. He did his job admirably.

Rikku

For Rikku my thinking was: HP, MP, SPR. She's a tough character with good defensive equipment and amazing ceilings for her pots. Of course mine is far from maximum potted, but every bit helps. The Army Shield is a great piece for defense with its Spirit (and even some lighting resist for Electromagnetic Burst in case I had to go that route). The Domination Rings were essential to the plan, because not only did they boost her HP and MP, they gave her more SPR/DEF and upped her ATK and MAG, too. Why would those stats be relevant, you ask? Because of one little selection I'm most proud of in this whole operation: Osmose Blade!. A hybrid attack--my napkin math suggested Rikku could steal between 80-120 mp from Aigaion and since he is constantly resurrecting himself and has oodles of MP anyway, if we can't find a good battery we'll just drain the mana. Osmose Blade was a huge win for this fight, because she could manage her own MP without the need of other characters. She had plenty of free turns, and this let her use Bushido Freedom (the 4th "empty" slot on the build calculator) to clear off the boss's buffs and HyperNulAll to keep the damage to a minimum. In my opinion this is not a game about the bleeding edge of minmaxing damage; this is a game about having the right things in your toolbox. Rikku checks a lot of boxes and I’m very pleased with how she performed in this battle.

Rem

Rem was much more straightforward. Her gear reflects a defensive set up since I wasn’t that interested in trying to get a lot of damage out her. With Armor Eraser and her Boomerang Stacked she could hit for ~125k before chaining bonus, so I thought this would be a nice bonus but not what she should be geared for. For most of the fight she just needed to be on repeat casting AutoLife on my friend’s Aileen. (Although I’d read that people used WoL to use Brave Presence, it made no sense to me that if someone was going to die anyway that you’d want two characters’ turns being used up when only one would do... I get that you might want to preserve the buffs on your attacker but that was not the case for me). Rem is another perfect puzzle piece for this fight because she counters the boss’s biggest threat (via Undying Wish) in phase 1, and in phases 2 and 3 she can either take over healing or take on the job of the finisher. With Ramuh she can even dual-cast dispel if necessary supposing Rikku is unavailable to Bushido. She summoned Ramuh during phase 2 before the Right Hand died for the first time, and other than that threw out Dagger Boomerangs, Curajas, and Dispels as needed.

Y’Shtola

I tried the fight with Refia in my two previous attempts but ever since WoL came on the scene I’ve been missing a reliable SPR buff. Enter Y’Shtola. Equipped mostly with her event gear, I tried to buff her HP, DEF, and SPR, paying attention to her MP as well. Her event gear and TMR (I managed to get a second one during the Titan Raid event) cover those bases. I gave her Lakshmi (only level 30, but still 2*) thinking that she could use Demonic Whisper effectively with her 522 buffed spirit = 10 0.3x(522/2 212/10) = 94 mana over 3 turns. There were long periods of the fight where Cura sufficed for Curaja and for the most part her auto-refresh kept her mana high enough. I could set up defenses and sneak out a Demonic Whisper if it looked like her mana was starting to drop. She was defensively solid and I don’t think she ever died (except of course when I was full-team auto-resurrecting after World Destroyer).

Aileen

You’ll notice this is not a great Aileen. No TMR (I have the moogles, but I wasn’t convinced it was necessary), no MP pots, insufficient SPR pots... I remember reading not long ago that Aileen is a great future character but not very good right now because of her difficulty in chaining. I took that to heart and spent my resources elsewhere and was somewhat blindsided by Aigaion coming out when he did! I did the best I could, equipping her with Whitebrim and 2x Domination Ring to build up her MP, and I chose Adventurer II over Spirit of Freedom for the extra SPR. I used Laswell’s clothes to increase her SPR as well. She was able to reach 200 SPR and over 5k hp, so I thought she was up to the task. Crucially, she was equipped with Odin to be able to use Lance to keep her MP up.

Fiendleen

Now this is a good Aileen. Mew, if you read this, I’m forever in your debt for the numerous deaths I unfortunately subjected your character to. Proper pots, the necessary esper, good TMRs, ideal weaponry, and still managing as much HP/SPR effective health as my own Aileen, I knew this was the right character for the job.

Step 3: The Left Arm

Every unit had a job. Rem cast undying with on my Aileen friend (sorry, old friend)... and Rem was the one character who could take advantage of the repeat button, so she always went last. Warrior of Light cast Light is with Us! and Embolden when needed, Raised any character who died due to incidental damage, and otherwise just guarded. An hour or so into the fight I realized that the spikes in damage I was taking was when I got clumsy/lazy about my buffs and they fell off the same turn Aigaion buffed his attack. I experimented with using Brave Presence to counter this, so that every hit would go to WoL... but the key difference between Brave Presence and Light is With Us! is that your tank can easily be taking 4x damage with BP... no 50% mitigation from LiwU! and no 50% mitigation from guarding. I abandoned that strategy and got more serious about keeping my buffs up. The Aileens cast Piledriver if they had the mana, or Lance if they were low on mana (having approximately the same mana pool they always lanced on the same turn). The turns the Aileens lanced, Y’Shtola would use Demonic Whisper because they did not need to be healed that turn. If one Aileen was dead but not the other (due to attracting several auto-attacks or a buff fading on the tank... probabilistically in a fight of 100 turns it will happen) I would use Refreshing Tool to just generally improve the mana situation. Y’Shtola tried to keep up Shellga, would use Cura if it was enough, otherwise would use Curaja. If the team was wall over 50% hp and I knew Aigaion was not buffing himself next turn, I’d use Demonic Whisper on whoever looked to be slipping in MP. As for Rikku, she used Bushiso Freedom if the boss had buffed, she used Al Bhed potion occasionally if Y’Shtola could only Curaja once, and she mainly used Synthesis into HyperNulAll to keep damage down, and whenever she was free to, she cast Osmose Blade to keep up her MP.

So Aileen doesn’t chain the left arm properly. At least not on a Galaxy Core from 3 years ago when she’s in positions 5 and 6. This in itself was probably worth a restart and I would have surely done that if I had believed this attempt had a good shot at being successful. This was only my third try on the boss and while I had a good feeling about it, I did not know I would be glued to my phone for the next four hours actually believing I was going to win when I casually started it up at 7:30 am that morning. If there are better positions for Aileen to chain from for the left arm, please let me know. I suffered through a 5-1-1-1-1-4 chain over and over as we gradually beat down the Left Arm (still scratching my brain for how two units could possibly even hit 1 chains repeatedly... I think I read somewhere that Aileen starts her dualwield attack before the first one is done or something?). Every once in maybe 20 it would do a full 19 chain and the health would just disappear... but it was more of a tease than anything else. Nothing I did could reproduce it. I tried the brilliant trick by /u/sgenial , but it would produce that pattern you see above, frustratingly well below damage potential. As the fight lengthened, my phone started to slow down. Eventually every time I attacked with my two Aileens it was as if I was pressing them simultaneously anyway due to the lag, so I couldn’t even try different timings. It was a long slog. I lost count of how many arms were dead around 5 or so and then suddenly it stopped coming back to life.

Step 4: The Right Arm

After giving my head a shake to ensure it was no mirage, the left arm didn’t come back. I started beating down the right arm... Hallelujah! 19 chains on the arm EVERY TIME!!! This phase was like a good dream; even the big heals the arm did barely slowed me down. Rikku continued to dispel, (more important now since the Right Arm buffs its DEF and the main body gets more attacks once the Right Arm has been killed once), Y’Shtola continued to use Shellga, Cura, Curaja, and Demonic Whisper if possible, Aileens played exactly as before, and WoL played like before. Now Rem could step up. She could start using Dagger Boomerang to add damage, she could dualcast dispel if I wanted to keep Rikku doing something else, she could Siphon Sphere for her own mana if needed, and she could even double Curaja, Raise, or Auto-Raise if needed. Talk about a Swiss Army Knife! She shone in this phase of the fight, patching whatever hole was needed or adding some good quality damage to the overkill nature of the 19-chain. She also used my Ramuh at some point in there to make sure I didn’t miss that achievement (I’ll never forgive myself for failing to use an esper on the True Titan the one and only time I beat it!)

Step 5: The Body

Then the right arm stopped spawning. I took a deep breath. Shellga, HyperNulAll, HyperMightyG, Light is With Us!, Embolden, Eccentrick. The party was made for this moment—all the defensive abilities we could want were on hand. I had Rikku to even do water damage to the body in the unusual event that her limit break was not charged and I needed to guard my way through a weakened World Destroyer. I didn’t know whether Electromagnetic Burst was an attack in addition to its usual battery of attacks or not, so I tried to make sure not to trigger it. I watched the phone intently, making sure not to screw up and miss “The Light is Concentrating” message. Normally I’d use a spreadsheet or track it or something but I feel like my turn counting “came of age” during this fight as it was just too long to use any tools. Once the first World Destroyer went off I took stock of the situation. My Aileens were not chaining to 19... they were doing the 5-1-1-1-1-4 annoying thing from before. But his HP was going down. It was just a matter of hanging in there. I guarded on WoL for the first World Destroyer and he survived with like 500 hp. Annoyance. It would have been better for him to have died and re-ressed with 80% hp; not going to do that again. So after a World Destroyer we: Light is With Us! Synthesis, Shellga Curaja, Piledriver, Piledriver, and Dagger Boomerang. Next turn we take stock of the situation and decide between Eccentrick or Embolden, and then double Piledriver, Dagger Boomerang, HyperNulAll, and Curajax2. My mana looked good (I made sure to be at mostly full before starting phase 3) and I thought if it came to that I’d use an elixir. I managed to Rikku LB every world destroyer and it took maybe 3 or 4 of them before I was looking at his final sliver of health and then: this. On this actual screen you’ll notice Rem, Y’Shtola, and WoL all still have auto-life on because the previous turn I experimented with hitting him with water before the World Destroyer to see what would happen: they survived; The other 3’s precise 80% hp tells you all you need to know.

He was beaten. The big bad robot, the boogie man of all our older siblings’ (read: JPN players) stories, the keeper of the arm, fell. Great fun, enjoyed the fight tremendously and just had to write this short novella/how-to recounting the experience because how many times in your video game career have you beaten a boss that took this much focus and energy! If for nothing else I want to read this post again in 5 years and think who the hell was this person.

Oops Moments

Fat Finger.

Sometime around hour 2.5 I accidentally meleed with Rem, and because WoL was right above Rem, I accidentally meleed with him, too (I think I was trying to slide right on Rem but the phone slipped in my hand). If you recall, he was equipped with a lightning weapon. I was targeting the Left Arm. Actually that part of it didn’t even enter my mind, and it’s only a stroke of luck that my plan to get around screwing up Rem’s turn also solved the lightning attack issue—I changed target to the body and attacked with both Aileens and even Y’Shtola to generate some crystals; enough to more than power up Rikku’s LB, which she cast and saved the day. I was more than a little taken aback that “the left arm exploded” message came up, but it all worked out okay with the auto-res, and two turns later we were back on track.

Rem casts Osmose.

I knew he had reflect on. I’d processed it. I’d worked around it. It had been part of my planning. She could have used Siphon Sphere. She should have used Siphon Sphere. There was no reason not to use Siphon Sphere except some small stupid instinct to flourish the esper I’d so brilliantly chosen. So yeah, she Osmose’d Y’Shtola... filled her mana up and drained Y’Sthola completely dry. Thank heavens Y’Sh had already acted that turn. Next turn Rem healed while Y’Sthola snuck out a Demonic Whisper on herself and that crisis was avoided. This was during P2 so I didn’t have to worry about the Left Hook.

Conclusion

I will be trying it again eventually with a friend Tilith (or if I get my own Tilith, or if some other 'no magic' healer becomes available) to try to get that last achievement... but I don’t think it will be until I find a way to chain the Left Arm more reliably. That was the kind of experience you only need to do once. I’d also like to take a crack at beating down the body first—if there is a positioning for Aileen to chain the body I could see that happening now that I’m armed with the fist and my Aileen can get 900 ATK.

If you read this far, thanks; I appreciate it. If you have any questions or insights into this fight I’d love to discuss them... this was the most fun I’ve had with the game and I think this community of players is wonderful. I’d love to hear about how you did it or what clever things you thought of to solve the fight’s problems. Cheers!