To get the Luxurious Chest on the sky south of Elynas, you need to complete a Precious Chest puzzle on the ground. In addition to the Luxurious Chest, you will also get 1 Hydroculus just on top of the chest.
➥ First, teleport to Elynas farthest south waypoint near a broken bridge and head northwest. You will see a sealed Precious Chest with Ruin Hunter guarding it.
➥ Defeat it first so you can solve the puzzle in peace later. There’s also a Ruin Guard near that location so it’s better if you kill that one too after this.
➥ There are 3 Seelies you need to find, and you can use Elemental Sight to see the direction of each Seelie.
➥ The first one is at the west of the Precious Chest inside a building. Once you get near the entrance, the gate will fall down so you need to climb the building to the top so you can enter it through the opening on the roof. Once the Seelie starts moving, the gate will get back up so you can follow the Seelie until it reaches the destination.
➥ The second Seelie is inside a building north of the Precious Chest locked by a Pneuma/Ousia puzzle. Climb the building to the second floor to find Mek enemies and defeat them. Two of them will drop Pneuma/Ousia Blocks and you need to use it to change the Relay to Neutral state.
➥ Once the locked gate is open, enter the building and you can see a Seelie in the next room. Follow it until the end.
➥ The third Seelie is at the southeast of the Precious Chest. For this one, you need to gather 3 Hydrograna to get Hydro Core and light up all 3 Water Volume Detection Crystals. After that, the gate below you will open and you can get the last Seelie.
➥ Claim the Precious Chest first to spawn a bubble teleport and you can enter it to reach the waterplanks in the sky.
➥ Once you reach the waterplank, your character will automatically get buffed so their jumping ability is increased. You can jump to the next waterplank carefully until you reach the Luxurious Chest and the Hydroculus.