You meet this boss towards the end of chapter seven. It attacks you along with two dark wraths. Aurora is the best character for this fight. Her light attacks deal the most damage to the ghosts. When you get rid of them, switch to Rubella. This will let you deal good damage with Finn's magic and heal your characters with Rubella.
This boss can slow your turns down and paralyze your characters. You have to be careful - if you don't kill the two specters, they will constantly cancel your turns. This may lead to losing one of your warriors. The Griffon itself is best dealt with fire or electricity. It can deal damage to all characters and even resurrect its friends.