We need to do something about Tanks and Outgoing Damage from Bosses

[YukioKobayashi]

Idk where you got those numbers from, but I highly suggest progging the ultimate and talk from your experience there, yes? Wave Cannon is a line stack which hits the first 2 targets in front for up to 200-250k unmitigated so 1 tank has to use invulnerability so healers can focus resources on only 1 tank and the party and here a kicker - roughly 40s before Wave Cannon 1 and 40s after Wave Cannon 2 both tanks will have to mit for Cosmo dive which is another heavy hit tankbuster. Yeah, 27k more hp from the old WAR trait would help tanking autos from the boss for they hit around 35-40k per auto but not much if any for the tankbusters. Healers and tanks resources are heavily used in P6 so idk how leaving a WAR with only rampart and vengeance as mits for they have very long cooldowns and take away its short cooldown's mitigation property and covering skill like nascent flash would ever make it viable and not a deadweight for that fight and i still haven't mentioned DSR and FRU yet. In FRU currently with all its kit WAR still needs help from co-tank and healers, now imagine with only 2 mits with long cooldowns and how that would do to WAR.

200k to 250k - your own 2min - the amount of party shields and mit = roughly 75k without Stem the Flow.
10% would reduce considering the diminishing returns 15k at most. Means the old Warrior MaxHP buff would be better there. You dont need to be a rocket scientist to see that.
You yourself took 71k there without Aquaveil 15%, Benesion ~ 10k Shield, Haima/Panhaima, Holos or Temperance. Your healers were simply not pressing buttons except E.Prognosis and Kera. Your WHM had one Aquaveil and one Benesion in the entire final phase. If you still think Warrior is the problem why you took so much damage there then sorry.. I cant help you.