I'm sorry to be the bearer of bad news, but if you've already completed the final Garm mission, there's no way to ever do the mission again. The tip you refer to only helps if you have killed the targets, but have not yet returned to Garm and spoken to him to get your reward.
In order to do the final mission over and over, you have to kill the targets, then speak tell the woman outside the capitol (or any mission-giving NPC) and tell her you'd rather work for her. This cancels the Garm mission before you've spoken to him, so now when you go speak to him you'll get the mission to do over again. This way people can repeat the final mission over and over again to kill the Dark Troopers and try to loot a holocron. However, to my knowledge, once you have completed the final mission by speaking to Garm you can never get the mission again. Hope this all helps, and good luck.