The battle is fought in five parts. The first four parts have 1,600 HP each, and although there is no visual transition between them, they each have minor differences in their stats. After defeating the fourth part, the monster fades like a defeated enemy and is revived.
Archeoaevis will sometimes use Wall Change to change its weakness. Its most powerful attack in his first form is Blaze that does around 250 damage to each party member, and inflicts Sap. Archeoaevis's only other random attack is Thunder, a level one spell that deals negligible damage, though it hits the whole party.
Once defeated, Archeoaevis reappears and casts Maelstrom, its strongest attack, reducing everyone to HP Critical status. After it reappears, it becomes immune to most forms of magic, but spells like 1000 Needles and Goblin Punch are still effective.
In his final form (after he has faded like a defeated enemy and been revived) Archeoaevis is on level 20, and so is susceptible to the Blue MagicLevel 5 Death.
Archeoaevis can be easily killed by the Blue Magic 1000 Needles. It takes 8–9 turns to kill him, but this will be easier with the party of four Blue Mages, or any job with the Blue Magic command equipped.