Dia is not fightable in Luminous Arc 2, but in Luminous Arc 3 she can be fought in hotspring battles.
Luminous Arc 3: Brilliant Spa (Chapter 23-25)Edit
StrategyEditYu has extremely high magic then they're completely immune to magical attack. Instead, focus on physical attack.
Both Glen and Levi shouldn't have any problem with just simple attack due to their decent technique. Ashley can uses Glorious Cross while Dino should uses Mind Projection. One can also have Lyra or someone gave Heine Drive Soda and lay waste with his Wood Huricane to easily destroy mop of Light Kopins. It's not recommend to attack them with Heine and Lyra directly due to their lower technique and lack of 100% attack.
While Dia is less powerful than Sadie, she shouldn't be overlooked. Shine Bullet and Photon Axis can put Ashley and Dino to critical status while Rev-Ray will target in 13 panels area and deal devastating damage against your party. She also has higher defense and evasion than Sadie so if you're low on level then your attack will miss. You can use Unison with Lyra or just attack with Lyra's Heaven Pillar to finish her off.