I dealt with this earlier today. Live chat a GM and they'll help you through it all. The customer service in this game is absolutely amazing
You have to party with someone else and do it and it will give you credit. Someone told me and it worked so yeah or I also heard the cut scene comes before he gets to 0 hp so you have to put a DoT on him to kill him
There's an Answer on this in the Tera Knowledgebase on here.
You (may) need a party to complete the quest and you have to hit first and do over 51% damage.
I did solo it as a warrior. I played conservative on my first run and failed to complete the quest. Had to reset the instance once, thought I killed it in good time. Soloed it the second time, made sure I hit first and just went balls to the wall damage-wise. Completed it and got credit before the cutscene started.
Definitely doable for a dps class, wouldn't relish the thought of a mystic or other low-dps throughput class trying to solo it.