I’m not sure if it was ment to be done the way it is.
I was fighting the pirate hydra queen in Bloodtide Coast. I had to do it twice to get achievement. For the first time when she was on low HP I started ressurecting a downed player next to me. Before I was dealing damage to her all the time. I didn’t get the achievement.
Me and my friends noticed that to get this boss achievement you should still hit her while she has no HP and gets the invulnerable state.
Please fix or at least inform us if this was a bug or a feature.