Tried again, 6 chars in. 4 chars had both dots running 2 chars 1, when Zland stopped fighting.
At this time we have been in for more than 6 minutes, we decided to try with low dps this time.
Waited until dots run their time once Zland stopped fighting.. Opened chest, no char got the achievement.
Yesterday tried similar, but opened chest while 2 dots where running, no achievement.
Day before we tried to get the dots to run out while holding Zland, but he recast before they run their course.
I don't understand which conditions need to be fulfilled, to get this achievement.
Have the dots to run out, while still fighting Zlandicar? Maybe half the group should hide behind a pillar to avoid the refreshment of the dots, so the dots can run their course while Zland is still fighting?