From the information I have gathered -> 15 seconds after the encounter starts, and then every 70 seconds boss sends in 4 players. The combination of roles seems to be the following:
- 1 Tank, 1 Healer, 2 DPS
- 0 Tanks, 1 Healer, 3 DPS
- 1 Tank, 0 Healers, 3 DPS
- 0 Tanks, 0 Healers, 4 DPS
After checking multiple videos, it looks like the encounter tries to cleanse 1 tank, 1 healer and 2 dps in an ideal case (except for first round -> tank is not sent here).
Targets for each group are selected based on their corruption level (the higher the corruption, the higher the chance you will be selected).
If there are no tanks or healers to be selected, these slots are filled by DPS (only with condition that there is someone remaining with corruption).
In case less than 5 players are alive -> nobody is selected (to prevent the encounter from evading)