So'sanuwa/Ul'sanuwa in Bis EX just hit like a truck. They frequently cast Tidal Wave/Dry Fin, which hits for 6~7k unmitigated, and do 1~2k-ish autos, on top of random 800-ish damage from Vacuum Wave. Fortunately, they don't do any of this while casting Dead Water and once this phase is over there's very little tank damage going out for the rest of the fight.

I believe Tidal Wave/Dry Fin are actually magical, so Dark Mind would be good for them as long as you have the rhythm down.

I've had DRK/PLD/WAR co-tanks and they all seem to be as squishy as the last. It's just how that phase of the fight works.