How to Complete The Ritual
If you do the quest I've Been Had, Kupo! before this quest, then you will not lose your judge at the beginning of the battle. Otherwise, Illua will get rid of your judge for the duration of the battle, meaning that you can ignore the law, but it also means that if any of your units are KOed, they can't be brought back during the battle.
The objective of this quest is to defeat Illua, so you can ignore the other enemies if you wish.
After the battle, you can go back to the mainland by going to the southern area of Zellea. If you want to return to Zellea, go to Lezaford's Cottage in the Aldanna Range.