Aura Mastery empowers your active Devotion Aura, providing a 12% raid-wide damage reduction for the duration. It also empowers whichever other aura is running, most usefully Concentration Aura, where it grants the raid immunity to interrupts and silences.

It is raid-wide damage reduction, making it uniquely strong when other DR requires a stacked raid.

Cover damage events where the raid is stacked with the ground-based DR options first (Anti-Magic Zone, Darkness, Power Word: Barrier), and save Aura Mastery for spread phases, soaks, or anything where the raid can't collapse onto a ground effect.

With the short duration, time the cast tightly to the damage spike rather than pre-casting too early.

With Ringing of the Heavens the timing shifts a little. The damage reduction wants to be up before the hit, but the free Divine Toll is wasted if the raid is still full. On damage that rolls out over several seconds, casting a couple of seconds in gets both: the reduction still covers most of the window, and the Divine Toll has real health to put back.