Eight-Armed Gilgamesh is one of the two secret bosses in the game, and can only be encountered on a third playthrough or greater. He appears via portals on the overworld, where he'll draw in a random character to fight him in an one-on-one battle. Should you win, you will receive that character's ultimate weapon, but if you lose, Gilgamesh will take your character's equipped weapon. So be prepared before you even try.
It's not exactly clear what makes the portals appear, but they can show up as early as when you exit the Deserted House after clearing the mission in Chapter 4. One theory is that you need to defeat Gilgamesh on three different occasions, which are in Chapter 6, Chapter 8 and once more in Machina's Struggle on NG+. In fact, that's ideal, as the first portal appears in Old Lorica and you will be able to see it when you are forced to that region.
You will find the portals at the following four locations, which are listed in the order that they appear, starting over from the beginning once you've hit all four portals:
- Old Lorica region (by Black Tortoise Crevasse).
- Azurr District (near imperial capital).
- Rilochy Province (near royal palace).
- Innsmouth Region.
The battle with Gilgamesh still plays out similar to previous ones, except he is a lot stronger (level 65) and has more moves at his disposal. What is the same is that your attacks will do little damage by themselves and you will need to rely on Breaksights in order to actually defeat Gilgamesh.
His attacks include:
- Shooting projectiles at you.
- Charging you while swinging swords, then hitting the ground and causing a small shockwave around him.
- His weapons will glow and float out from his hands, then shoot into the sky, with them landing where you are located. Dodge quickly away from the weapons (you might take some damage).
- He will stab with his swords, then do the same overhead slam mentioned before.
- Gilgamesh can also do the overhead smash by itself.
- He will do a series of slashes, before finishing off with a projectile blast.
Remember that whenever you hit a Breaksight, you want to stop attacking, as if you do keep attacking, then you won't get the Breaksight streak whenever Gilgamesh kneels down. There are no new moves added to his repertoire at any point, so you simply need to dodge all of the attacks described above. Patience is the key to any fight with Gilgamesh.