When you have the link, run away to not let the Goobbue's stomach absorb you. If you are eaten, your party members need to damage the stomach til you're released or you would die.
There are a bunch of moth adds in the fight, the adds will add debuffs to party members. The boss will attack the player with the most debuff. AoE the adds if possible.
There are 8 doors divided into pairs of 2 in the boss room. The strategy is to have each member of your team responsible for one symbol, the blue goobbue, red demon, the light green dragon-like, and the dark green moth. Each symbol will appear above 2 doors. Make a progression of who will open their doors first, this is important for later. When you open both doors you get sucked into a void and you become immune to attacks, When Diabolos casts Ruinous Omen, you need to use the doors or the party will wipe. At the beginning of the fight have one of your party members open his first door, and leave it open, when you get to about 60% of Diabolos health, he will go to the center of the arena and start to cast Ruinous Omen, once it gets about halfway through the cast bar have the party member open his matching door. If timed right you will get into the void as soon as he casts Ruinous Omen and you won't take any damage. If your damage is halfway decent you should only see Ruinous Omen twice, so two of your party members will have to do it. Remember, you need to look around and memorize the positions of the 2 matching doors you're responsible for.
Avoid the orbs when they appear, they apply heavy and deal more damage as the debuff gets stacked on the party.
Note: have your healer take the job of opening the doors instead of the tank and dps. Open the first door when you can and stand near the matching door if needed. If done right, the group wouldn't be taking enough damage for the healer to be doing much healing.
Note: When given the buff that makes you immune to attacks, aggro may change if you have a summoner or scholar with a pet out. Diabolos may switch to the pet and they do not get the immunity buff, so summoners could use Titan at that point.
Born in magic, the ancient city of Amdapor lived in splendor and died in agony. As the Fifth Astral Era gave way to the Sixth and the ensuing catastrophe, the agent of Amdapor's destruction slept. Now, the seal that has held so long is softening like windfallen fruit─and what blind worms writhe behind the skin of the dead city?