When i beat Angra Mainyu, I had a berserker, gunner and an alchemist. First i went to the 40th floor of the cloister of trials and got the catnip accessory, gave it to Yuna (gunner) and went into battle. Make the alchemist keep mixing holy water and any gem (which makes white hole and deals up to 9999 damage) and make yuna keep doing trigger happy to the mainyu part, keep doing this for about 5 times, and as soon as you kill mainyu, the rest of it dies aswell. You also need to level up a bit, (i couldn't beat it until i was on level 88, 89 + 90.) Because it kept killing me with perditions flame because i didn't have many hit points.
hope it helps a bit.