Took 45 seconds, I know people have downed him faster, just showcasing Celestial Nighthawk damage on the boss and how easy it made it. I'm now wondering what 3 Celestial Nighthawk would do, or 2 with weapons of light.
...and before anyone starts with "that wasn't 45 seconds" I'm roughly counting from the 1st rocket fired, from 100% health to down time... it may even be a little faster.
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Completed it with 2 Gjallarhorns and the third dude had the DO rocket. No Titan on our team. What a pain in the ass that was.