**Spoilers for final Boss Strat**
It's recommended to be at level 7 or 8 at least. Reserve Aireen strictly for keeping up HP and MP and the protections spells to mitigate the damage. Shimmer can also heal with her AoE milk healing skill if everyone's looking bad. Rose and Frez specifically are the damage dealers unless thing's are looking bad, heal themselves.
at 50% HP, she'll start to use use a skill that'll make her invulnerable unless you burn her with Shimmer's fire spell. Continue as normal once the spell is removed. Be careful not to let your party members die. Keep them alive, otherwise it'll be difficult to catch your momentum again.