You start off controlling Melia which is easy enough. To use her arts is different to the other main characters. She can use up to three elements which builds her magic in the middle. She must use all three or she won't be able to rebuild her magic again. If the talent art turns white, you can use Mind Blast and Burst Air to weaken her enemies. She can also use Starlight Kick and other arts as well. The middle art can only be used once you chose an element like water for example.
At the Ceremony Hall, you find out she's 80% Homs and 20% High Entia. Even though she's fit to rule, Tyrea interrupts and what happens next is comes after the rest of the team comes.
On the other hand, the team came in and Reyn push a button which made everyone fall into a water pit! So unfortunately, your team has to take a long way to reach Melia. So ideally when you reach the circular area, go down or jump down to the water if you're impatient. Move across using the climbable walls and start attacking the metal monster creature. Get to the other side and find a path which leads to the top which is pass the bridge, through the hallway on the bottom floor at the other end. Continue up attacking metal monsters until you find the switch that will open the way.
If you have found Melia, battles on. It's your team vs level 38 Solidum Telethia and Tyrea. The Telethia would be easy but Tyrea can take some time since Shulk's Monado has a mild effect on her.
Even though Yumea's under investigation, Shulk's team enjoys the spectacle and learn more about the Monado and whether Sorean will allow his team to get to Prison Island. Anyway, your team should head to the garden area which is on the opposite side after the cutscene with Shulk. Your team meets Melia and Shulk suddenly realises that the Emperor is already at the place. Go to the Audience Chamber which is on the right side when you're elevated down from the gardens, on the top of the stairs to have another cutscene involving Melia's decision to go to Prison Island.
On Eryth Sea, to get to Prison Island, there's two places to go: Soltnar Seal Island and Khatorl Seal Island. If you have to take out monsters, do so then teleport back Central Seal Island. A level 40 Skyray comes down to battle against your team. Finsh any remaining quests before going through this moment of what happens next.