Silence is Golden

SILENTIUM EST AUREUM.

The sound bar is increased by getting hit by practically any of Atramedes' AoE abilities (Most of which are avoidable). If it gets maxed out, he'll immediately target the player(s) in question with a rapid barrage of fireballs that will kill them off almost instantly. To make matters worse, some of his abilities pursue them at a greater speed depending on how high it is.

There's gongs that can be used to stun him and reset the sound bar of every raid member, but those are mainly meant to be used in order to interrupt a powerful channeled AoE he casts periodically that causes a raid wiping amount of damage, as well as to reset the speed of the flame breath he chases players with during the air phase.

A group with decent DPS can down him well before they run out of gongs, plus the unavoidable sound increases from Modulation alone won't push you over the 50% mark before you'd need to use a gong to stop Searing Flame or Roaring Flame Breath, so even if some mistakes are you made, you can potentially use some gongs before either of those two abilities to keep yourself low enough to get the achievement.

Sound bar... color me intrigued.

I'm definitelly not battle shouting in this fight.