Getting the Sacred Eye shield:
During the first assault on Cynas, you make several parties, one that must include Asad, One Crodechilde, One Diulf, and you are stuck with Liu in the hero's party. Simply don't add anyone else to Asad, Diulf, Crodechilde's parties, and they should get trounced pretty soundly. When they fall, the elevator to the next level will not work, and you will have to walk past it into areas that you'd not normally be able to access - it's pretty linear for the most part - the treasure boxes will be visible or on the path to the switches. Along the way to the switch, you will encounter treasure boxes, one of which will be a fenrir sword, and another of which will be the sacred eye shield, (which can be coupled with the Beast God sword to make your hero a very viable melee/magic hybrid using dying wish, or put on Marica with a prominence to make her a very healer/nuker hybrid.)
You won't loose the characters that get defeated, they will be waiting a ways from the switch all beat up and sad they failed. Nor will you have to fight their "boss" - once you flip the switch yourself the elevators will work normally.