it's basically just a DEF buff for Illuyanka throughout the entire battle.
After you attack for the first time, she stacks a DEF buff that cuts your dmg x3.. the only way to get rid of that is using CM and Awakened Sol, I guess... (in the battle it seemed the team had really low atk with those buffs, even with x2 Kirins...)
so better to use all abilities in turn 1 of course.

I guess for expert/ult it doesn't matter too much, but for Rag, i'd honestly recommend just attacking once with no debuffs, then waiting out the CD for her buffs to go... and then again, when the boss reaches less than half of her HP, she'll stack another 3 DEF buffs