Its called Tail Screw and it targets a random player (other than the tank I believe). Shortly before the attack he will do a slime attack which inflicts a debuff. If this debuff is not Esuna'd or Leeched, the Tail Screw will one shot that member. It is unavoidable damage but without the debuff, it will leave the member with like 1% health. So just as soon as you see the slimes go out, dispel all the debuffs and you are good to go.