I got one-shot by this boss at a certain point twice and I had no idea why. Turns out you need to stay relatively close to him, otherwise he is angry that you are too far away (which you want to be as Archer...) and he then just one-shots you at some point.

Stay inside the yellow circle. If you leave the circle, the boss will restore health etc. If you leave the circle 3 times he will one shot you.

Same problem here. I've been trying to kill him for 2 hours. I stay in the yellow circle. No chance. Might play again when the event is over.