You will meet Cordelia at the end of chapter eight. She will morph into a huge black dragon. She also has two dark ghosts that help her. She's able to slow down and paralyze your characters. Additionally, she can replenish the health of hers and her allies. Aurora and Oengus is a smart choice for this fight. The light attacks of the protagonist will quickly deal with the ghosts.
The boss itself should be attacked with regular attacks. You might want to slow down her turn cooldown with a potion. If you can't play with Aurora, you can also try Finn. His fire attacks are pretty effective against the ghosts, but not against the dragon.