With help from Garlond Ironworks and the Zundu, you head to the island that will be used as bait for Bismarck.
How to Unlock
The Limitless Blue (Hard) 8-player trial is unlocked by the level 57 Main Scenario Quest Bolt, Chain, and Island.
There are two Dragonkillers, one on the north end of the island and one on the south. In addition, there is a Magitek Field Generator in the middle of the island. These need to be used at specific times in the battle, as will be explained below.
There are no walls, and if you fall off of the edge, you will die and a healer will need to revive you.
Move closer to Bismarck to start the battle. He will descend, then hit the island, inflicting everyone with a stun status. After this first attack, watch for Bismark to dive downward during the fight, and activate the Magitek Field Generator as soon as you see him doing so, to protect the island from being destroyed.
Throughout the battle, attack the Zundu enemies, and don't let yourself get hit by their line attacks, or you will get knocked back, which could result in you falling off of the island and needing to be revived by a healer.
Watch for the Dragonkillers on the north and south ends of the island to become interactable. A message will be displayed when they are ready. Both Dragonkillers need to be activated as soon as possible, so make sure a player goes to each one, and activate them yourself if nobody else is doing so.
Once both Dragonkillers have been fired, Bismarck will be pulled toward the island. When both Dragonkillers have been fired, all players should stand on the bright green area of the eastern edge of the island. As soon as Bismarck is up against the east side of the island, all players need to jump onto Bismarck before attacking the Chitin Carapace on Bismark's body. You cannot damage it if you are not standing on Bismarck. Once you are standing on Bismarck, attack the Chitin Carapace for as long as possible. Bismarck will eventually throw everyone back onto the island; no need to jump back onto the island yourself.
After you destroy the Chitin Carapace, Bismarck summons an Ul Sanuwa and a So Sanuwa. One tank should pull enmity on one of them, while the other tank pulls enmity on the other, and the tanks must keep the Sanuwas away from each other or they will become stronger. While battling the Sanuwas, be sure to dodge the small tornadoes that move around the island. If they hit you, you will be launched upward.
After a while, Bismarck will reappear and cast Breach Blast, which hits all players. This attack does more damage if you did not defeat the Sanuwas in time, and you may not survive if that is the case. This destroys the Magitek Field Generator, so you don't have to worry about shielding the island anymore.
More monsters will appear, and can be dealt with as in the previous phase. As before, watch for the Dragonkillers on the north and south ends of the island to become interactable, and use them as before to pull Bismarck to the island. As before, all players need to jump onto Bismarck before attacking the Corona on Bismarck's back.
Vapor Bubbles will appear, and you should destroy them when the current weather (visible in the indicator next to the minimap) is rain (blue), but ignore them when the current weather is thunderstorms (purple). The Vapor Bubbles will drop water attacks when it is raining, but they will absorb lightning and prevent it from hitting players if it is thunderstorms.
Completing the Quest
Defeating Bismarck will complete one of the quest objectives for the Main Scenario Quest Bolt, Chain, and Island. There will be a cutscene. This one is important, so it is recommended to watch it either now or via the Unending Journey in an inn room.