Fire resist gear and massive DPS - there’s the secret. Put dampen magic on your mana users right before the fight - they can stand on a Son and not take dmg. If you don’t have Rags down to less than 40% before he submerges, you don’t have enough DPS and you probably won’t get him. Collapse into a single point when the Sons show up. Freeze trap them, snare them, whatever to keep them together and burn them down. Have your MT back in position right after the Sons go down or before he comes back or he will start throwing 6000hp fire balls around. Always have 2 tanks on him in case one gets knocked back. If you can beat Domo, you will eventually beat Rags. He’s nowhere near as technical a fight as Domo - he’s just a gear dependent fight.