Quote Originally Posted by Kitty View Post
Ultimate Battle - Normal Overdrive randomly attacks 7 times, Raging Overdrive deals overall damage to all allies, and decreases their two-stage attack probability. (Special Skill can cut your DMG, so pretty much a DEF buff to Illuyanka)

Ragnarok Battle - Normal Overdrive randomly attacks 9 times. Raging Overdrive deals overall damage to all allies, and decreases their three-stage attack probability. (same special skill.)

As it states in the wiki, there is nothing that special to mention, so this is a pretty easy and straightforward advent battle, which is good.
The only thing to worry about is the special skill, which can be used at any time. It shouldn't be too much of a problem in experts/ultimates.
It's NOT 'any time'. It's a new mechanic. 'Trigger / Conditional Specials'. Almost all new contents from now on will have trigger specials with specific conditions.

Illuyanka will buff herself with THREE (which you failed to mention) dmg cut shields on the '1st turn' and once more 'after 50% hp or less'.