This is, quite possibly, the easiest and laziest way to beat Ryth at a very high OG level.
Solar Trans- LX (light): elemental attack gem (+400 or +500), On Combo5 Continue combo
Vile Shield: +200/250 attack gems (both slots), elemental defense
Aran: Equip with attack gems
Aegis Armor: Magic +200/250, Titan Break +2
Since a high level Ryth has the potential to kill you in one hit, no matter how much SP you put in health, here is the SP build:
Put 100 points into shield. This should give you ample blocking ability.
Put the rest into attack.
What you need to do is, quite simply, block every single one of his attacks. They are ALL blockable. When your magic regens, use that Shield 5 spell. This can restore over 100 points of shield.
I faced a level 21,000 Ryth with this setup, but I didn't even use an On Combo5 multiplier and I only had 1 rare gem, and I still beat him on my 1st try.