Here's a crude graphic of how I do Heroic + Been Waiting each week for my mount farm group.
Raid is red, melee don't need to be near the tank. Have ranged get the boss to 75% or so then stop.
You rotate counter clockwise around the platform moving for each shadow trap.
First two plagues run out, every other one runs in. Don't AoE or kill ghouls etc.
By the time you get from the right side of the portal to the left, going around the ring you'll have 30 stacks. Done.