I'm assuming you're at Gnosis Cathedral Ship.

That's not a really tough battle... Gargoyle is weak to lightning, and Oudogogues are weak to slash, and they have about 1/3 the HP of Gargoyle.

Take out the Oudogoges first, and yeah, Gargoyle will power-up and heal, but after you finish off both Oudogoges, you just need to focus on the main boss. Heal, Attack.