I see this every time I play this strike. The fat bastard busts through the doors, spraying his gun, and people immediately unload all their heavy ammo. Have you not played the strike before? You do realise he has an abnormaly long period of invulnerability, right?
By the way, his invulnerability wears off when the door behind him finishes closing. If you're still not sure, test with your primary before wasting your heavy.
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You guys have it all wrong. Before the door closes, get in the center of the circle and face Valus. Tap UP/DOWN/SIDEWAYS/DIAGONAL twice, this will initiate a dialogue. It should go like this: VALUS: GGGRRRRMMMMGGRRMMMAARRGH! You: Hello good Sir! Beautiful day on Mars isn't it. Valus: MMMGGRRRMWORGGG! You: Heard your mom calling you from down below Valus: MmrrGrr? You: No really, not kidding The large Cabal will then head to the edge of the cliff to go check for his mommie, you and your fellow Guardians will then gently push him off to his death, works every time.