Instructions
Open the fight with magic attacks, as Deathgaze begins the fight with "Paling" and "Reverse" cast on itself. Normal attacks will heal an opponent that has cast "Reverse" on itself. "Paling" negates physical attacks.
Focus on "Earth" attacks, such as those with the "Athena Rod" and "Artemis Arrows." There are no spells that do Earth damage. This is the only element Deathgaze is weak to. Cast "Shell" on your party members so they cannot be killed in one hit by "Crushing Fangs."
For a much faster battle, see Step 3.
Cast "Reverse" on Deathgaze, and in the same turn, follow that up with "Renew" from a different character. This will leave Deathgaze extremely easy to defeat. "Reverse" causes white magic spells to cause damage and "Renew" is one of the most powerful cure spells, which fully restores allies. This will reduce Deathgaze to 1 HP. A simple "Cure" spell should kill it quickly.