Maybe he got it from the sevics prime the first strike boss. It's not in rotation on vanguard but maybe that is why it is rare because no one is doing that strike anymore.
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I've killed him four times with new players who have never killed him, but I didn't use any three of coins. He dropped really low light blues.
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Then maybe you need to use 3 of coins to get the exotic. They who has it is in the Rockets yards which I think is a hint to how he got it.