He doesn't attack at hundred fist speed. It -can- attack a maximum of eight times in one round. After the tentacles are wounded, it will attack less. I don't know for sure, but I'm almost certain it goes 8>6>4>2>1. Even after all tentacles are removed, it can hit for ~200 and upwards of ~600. (I had to poke some fun and let it loose on RL Scwall who was dualboxing 2 BLM, it literally one shot our tarutaru)
I'm positive you can tank as RDM/WAR...if you don't let it near you. Gravity doesn't build resistance and lasts longer than the recast timer. If you find yourself in a sticky situation, simply Bind it.
I attempted to solo this ( as RDM/WHM -_-;;; ) and had it at around 20% HP remaining before time ran out. If I go as /BLM, more INT, and actually use Chainspell...I plan on success.