You get here by using the DimensionFragment item during @Party (while questing).
The overall Attack level is insane. Perhaps it's best to leave DevilScallop, Golem, and Dragonlord for last, defeating the others first?
For Gigantes, it's not a problem if you know a skill to remove Tension (you can even defeat them directly by using PuppetString)… but taking a gamble and using a skill to lower their Accuracy is also an option.
First, let's introduce the two minions. Archfiend has an Attack level that's practically invincible from the start, as they already have Oath equipped. If they use DarkArtsSword in this state, it's a devastating blow. And if they use Oath to get 2x, you're practically dead.
GodOfDeath's attacks are mainly magic attacks, so their Attack level isn't so important, but their Agility is incredibly high. Physical attacks are extremely difficult to land.
And finally, the main boss—the true final boss of @Party—Catastrophe.
They start with an abnormally high Attack level of 750, and can get 2x with Oath. If they use Multifists, DarkArtsSword, or Omnislash with that kind of Attack level, someone's almost certain to die. It might be a good idea to have one of your party members equip AttackReflect at all times.
As a basic rule: immediately revive any party member that dies, keep AttackResist and NullPhysical active, and maintain near-full HP.
If the enemy's Attack level increases, it's best to lower it with skills like Seal, WeaponBreak, or DisruptiveWave.
It's best to eliminate the enemies in order, starting with GodOfDeath.
*Three treasure chests appear per player (only one additional chest will appear from the Merchant's bonus effect).