-( Story related, cannot be missed )-
Fighting Ariadne comes in three forms. The first stage is where you have to hurt the real Ariadne among the four that surround Gremlin. While you can simply start attacking them one at a time to knock them into an Invulnerable state, the one that is the real target is the one that is armored rather than simply shielded. After completing that phase, the second form involves collecting the three energy forms of the fake Ariadnes and shooting Gremlin's cell.
The third phase is the actual boss fight. I used Joule and Braddock (B being 20+ for the fight). Anyone with Shieldbuster specialization would be handy here, as she is protected by a shield before you can truly interrupt her. Once the shield is down, lure her into one of the target zones and have her perform her dash attack against one of the dioramas she has made, stunning her. This is when I changed over to Braddock and used her vulnerability special to quickly drop Ariadne's health down. You will have to repeat this three times before she will finally concede.