The Stone Vigil last boss should take no longer than 5 minutes to down. I've come into runs of that on last boss with about 8 minutes on the clock and rushed in to kill it in 4 minutes. The only two things that will kill you is not moving quick enough out of AoE's and eating too many or if the tank starts turning the boss towards the party and the ice breath hits everyone. You can eat an AoE by accident once and your healer can fix it but if you keep doing it you'll mess up your healer. For the healers themselves just avoid stuff and be ready when the boss starts charging up because it'll either do ice breath on the tank and you'll need to heal and clense a debuff or a stomp AoE so have medica ready. All you need to do is stay behind the boss and when it flies in the air pop sprint for some extra evasion speed if you are having problems. This is especially useful when the boss spams AoE's because you can just keep moving and never be hit.