tl;dr strategy:
Melee DPS - Cannons, LB3 near the end (even if it's needed.)
Tank/s - Adds, as stated, as like Senpai, the boss doesn't notice you nor give a **** about you so he needs no tank.
Ranged DPS (BLM/SMN) - Do their DPS thing and sprint to the Dragon Killer, they can also handle snares if need be.
Ranged DPS (BRD) - Stay on Boss, especially if highly geared.
Healers - Heal/DPS/Snares.
With tanks you can have one dedicated to Dragonkiller duty as well, but you got the general idea, it's just much easier to designate certain people to the mechanics then using LB3 (melee) on vishap near the end. Ideally, you want to go for the "75/45/15%" progress through the barriers, the lower the better but those are generally where you want to push and be around 5-10% near the end so the LB3/LB (depending on situation) can kill it off.
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