Warrior would be the best healer in the game. Warrior only party, have one Warrior cycle on cure duty at all times just wailing. Maybe put a Bard/RDM in at 99 for Haste/Marches.
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"Oh crap it 2 houred! Quick, beat the crap out of me! we can do this!"
From what I've seen, there's no limit to how many times it can proc. Next time You're FCing, pop Seigan and third eye while you have 20 mobs beating on you and you'll see that you can mitigate 2 full attack rounds from all 20 mobs on occasion. After the first 5-10 seconds, it becomes less likely to anticipate/counter, but during the first 5-10 seconds the chance is so high that a lot of the time it's almost guaranteed. It's probably the best defense against Hundred Fists by NMs too during that time.