The video guide was done on the second playthrough on Hard Mode, but you can wait for the 3rd playthrough if you wish. It was also done without any Adamant Armor or Onion Gear, for those who can't seem to luck out with tail grinding.

This guy can be found in New Game+ in one of two places. After you kill the Four Fiends gauntlet in the Giant of Babil, you can find Geryon in the same spot where the Four Fiends ambushed you. Or, if you take out the giant's CPU first, you can also find Geryon on Mount Ordeals where Cecil became a paladin.

This guy combines all the tricks of all of the Four Fiends, including Scarmiglione's Curse and Cursed Elegy, Cagnazzo's Tsunami, Barbariccia's Maelstrom, Lightning, and Stone Touch, and Rubicante's Inferno. Inferno is powerful enough to nearly one-shot your entire party, so make sure you have a couple Phoenix augments handy. With that, you also want someone with Bless to restore their MP after using it. Or, you can just equip everyone with Adamant Armor and Cursed Rings to absorb it all (obviously the easiest strategy), but if the RNG doesn't like you in that regard, just follow the video guide.

In order to access this bonus boss, you need to have Edge steal the Dark Matter item from Zeromus in a previous playthrough. Then, use it on the face-shaped rock on the Lunar Surface. The video guide was done on the second playthrough on Hard Mode, but you may also wait for the 3rd playthrough if you wish.

Make no mistake, this guy is HELL! Even with Adamant Armor his Light of Babil attack will still do 9999 damage to one party member! At least the armor negates Ninth Dimension, but what's worse is that he'll randomly counterattack with Laser Barrage dealing 4000-6000 damage to the entire party, which will pretty much kill everybody. With this in mind, you want at least two Phoenix augments going at once, and you want somebody to Bless them to restore their MP after being revived. Healing will do pretty much no good since all of Proto-Babil's attacks are pretty much OHKOs, so have Rosa constantly revive anybody who might fall after either of his main attacks. Whoever you gave the Limit Break augment to after the first playthrough (in my case Cecil, although Kain may work better in tandem with ????'s Love) will want to do most of the attacking, but you don't want to attack until you've Focused three times and activated Darkness. Then unleash for 60,000-75,000 damage.

You'll want to keep track in your head of how much damage you've done, because after you've done 280,000 damage in total (out of his 400,000 HP), Proto-Babil switches things up. At this point he starts counterattacking with Object 199, which always deals 9999 damage to one party member, he starts healing himself for 28,000 HP each time, and worst of all he starts using his ultimate attack, Divine Judgment, which does over 9000 damage to the entire party! In the DS version you could skip the second phase altogether by fighting him in New Game++ and applying the above process with two characters, but in the iOS/Steam version, you don't get a second Limit Break after NG+, making it much harder.

So how do you counteract this? First, hover around 270,000 damage done to Proto-Babil (just before the Phase 2 threshold). Next, make sure your two Phoenixes have MP to spare. Then, make sure your Limit Break is prepped and ready (three Focuses and Darkness). Once ready, have Edge Hide so he won't die from Divine Judgment. Then unleash your Limit Break, have your other main attacker spam attacks, have Rosa Dualcast Holy and Rydia Dualcast Flare, and have Edge throw Fuma Shurikens from off-screen. Just keep pelting him as much as you can as fast as you can before he has a chance to spam Divine Judgment, and with a little luck the prototype Giant will fall!