The mechanism you describe will not provide the same result. How could I possibly add the wisp loop trigger, that runs every 0.05 seconds into a 0.5 periodic event trigger? Each loop calculates and picks different stuff. If I combine some of them (that they probably have the same periodic event, e.g. 0.05 seconds), well, the pick on the wisps might end, while the other units, the dead ones that are dragged to the wisps' column should be still moving. I mean the trigger should stop for the wisps and keep itself on for the dead units, while I make a condition call to turn it off, once there are no units left in the wisps' group.
I really believe there are many triggers required, because of the spell's multiple sub-effects:
The casting trigger, the wisps' rotation, the one that counts the deaths to respectively add the dying unit in the trigger it should go, the drag of the dead units, their elevation and another trigger that works as a debug (the one with the timer) that kills the dragged units, in case the spell's duration is up, before they reach the column.