Your parry and dodge are out of balance. For a warrior, there is some value in having more parry than dodge because of the talent Hold The Line. However, I believe 5% is too much. I would aim for closer to 1-3%. This may bridge that last gap with being passively covered for ctc as well. Also, change your meta to the 81 stam and 1% block value. Also, not sure I would use pure stam gems until you are ctc cappped. Making some of those mastery+stam would likely cover any ctc you are missing.
As to the third platform, you would want to look in recount/skada/death notes/world of logs and see what actually killed you. The secret to single tanking the fight is the dps killing the mutated corruption prior to the bolt coming out (and getting the second impale). This is also affected by your strategy. If the elementium bolt is being slowed down on the third platform that you kill, then the bolt should be dead before it explodes. If not, your survivability is not the biggest issue. If you are not slowing the bolt on the third platform due to the order you down them, then you need a strat to specifically cover the huge damage from that on the tank while everyone else stacks away from the landing point and the ranged kill it ASAP.