Focus. If everybody focuses, calls out enemies, and maintains focus - provided they are sufficiently levelled, you will not die. Treat this as a test of teamwork, not of how many enemies you can kill before dying.
Try to build a team composition that works as well.
And preferably having all your gear levelled so you aren't doing shit damage.
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This. I sometimes run pugs on hard mode and these 29s were so focused on gear that they neglected their weapons. 300 or bust.
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A fully upgraded Gjallahorn = Fun times. We had 1 person with that during our hard raid against Atheon; he basically took down 1/5 of Atheon's health with two rounds (Time's vengeance + Weapons of light). Imagine every single member of the team having a maxed out Gjallahorn - because that's Atheon down in one round.