You are trolling if you are not using essences and actions in Delubrum

[Rein_eon_Osborne]

I've rarely seen BLM or RDM uses Essence of Watcher, eventhough it's really great paired with Chainspell.
And with the effect duration now the same with the CD timer, you basically get perma instacast (plus perma 45% damage buff).

The damage buff is actually separate from the 0 castime buff. So no, damage buff remains 30s long even w/ watcher.

I imagine the premade is mostly 1 tank and rest DPS\Healer, right?

Plausible. Still, Healers & Tanks in this content can actually pull off some monstrous degree of damage surpassing some legit DPS when done right.