"Epic Undertaking" is a veteran mode Falkreath Hold dungeon achievement one can get from the Horns of the Reach DLC pack.
Now the achievement reads "While fighting Deathlord Bjarfrud Skjoralmor in veteran Falkreath hold, have your group cleanse 15 corpses within the span of five seconds."
This achievement took our group a lot of tries because we simply couldn't figure out what in the Vivec's underwear was going on. How we weren't getting 15 corpses, how simply waiting while passively killing the two stronger adds that spawned every phase wasn't working, did I leave my oven on, so many questions plagued our minds until a nice fellow on the Twitch stream gave us a little hint.
Instead of killing the two adds that spawn every phase, your group needs to keep every single archer and melee enemy alive, they die very very easily to damage over time Area of Effect attacks so as you can see in this video, we're minding our positioning, taking the boss' health lower as we go in preparation to finish the achievement.
We also have someone else, the healer, slot a taunt to take the boss away while the tank stacks what they can on another side of the room.
Now the boss and the adds once they accumulate can hit pretty hard, so whoever has the taunt on the boss if it's a DPS or healer doing it, needs to roll dodge the heavy attacks.
I think it's probably easier in terms of add management and not getting one shot by multiple things at the same time for the tank to take care of the adds and a healer or DPS take care of the boss.
At least this is how we did it fairly easily. Eventually, because we really really sucked at doing a headcount, we made sure to wait a considerable amount of time and decided it was probably alright to kill everything.
Once we did, we had to make sure to fulfill the second component of the Epic Undertaking achievement which is to make sure we cleanse at least 15 of the corpse souls within five seconds of cleansing the first.
Due to how we stacked the adds, it was fairly simple for us to relay to each other when to use the synergy to cleanse the corpses.
Unlike many other achievements that pop up after the boss fight, if you do this achievement correctly, the Epic Undertaking achievement should pop up before you kill the boss.