Bound captain invincible?

Hi I'm a new player and I'm trying to power through this game's utter hostility towards new players. I was playing a mission inside the traveler. At some point I found some bound ennemies and a well of transcendance. I put myself in the well, got the buff, killed the ennemies, except their captain was still invincible? I had to just leave the game.

How am I supposed to kill him? thanks in advance.