Trial of sekhemas betraid me

I was in the trial of Sekhemas and i had Moment's peace buff while fighting the last boss, had him to 40% I also collected the dots in time so i did the mechanics.

But i shouldn't take dmg at all because of the "you will not take dmg until next room buff" but the ability when he is turning around actually will damaged me.

No I'm damn frustrated because this was the only chance for me to get the last ascendancy points.

I'm discouraged to try that stupid challenge again...

https://imgur.com/a/vvyYyo7

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