Grand Aevis is accompanied by two Dark Elementals that cast third tier elemental spells every turn,, and uses status attacks on the party; Blasterparalyzes a target, Evil Eye causes petrification, Poison Breath damages and inflicts Poison, and Zombie Powder turns a target into a Zombie. Grand Aevis also uses a target-all, Wind-elemental attack called Breath Wing, and Maelstrom that reduces a random target's HP to single digits. Additionally, it can use Paraclete, which may raise a KOed party member into a Zombie. If both Dark Elementals die, two more appear which can cast White Wind, healing Grand Aevis for close to 5,000 points.
Before the battle it is useful to equip everyone with an Angel Ring to protect against zombification.
The Grand Aevis appears with two Dark Elements that cast third tier elemental spells but the party can set up Reflect with Carbuncle. Before that the party should hold on just long enough to cast a few defensive/supportive spells (Golem, Mighty Guard, and Hastega are good).
With the spells being reflected back at the opponents, it may either cure them or hurt them. Since they do not cure more then the party can deal damage, it does not matter. White Wind is an effective healing method. Good offensive spells and attacks are Bahamut, Meteor, Holy, Flare, and Rapid Fire.
It is recommended to always leave one Dark Element alive to avoid them respawning and healing Grand Aevis.