Hey,
Steps:
1.Go as Blood DK in Bloodmaul Slag Mines
2.Glyph Dark Simulacrum for a smaller CD (optional but recommended)
3.On the last pack of trash mobs (Ogres) they have the Channel Flames ability which does a quite big amount of damage
4.Use Dark Simulacrum on it in order to steal it
5.Start the boss and use Channel Flames which you just stolen from the Ogres on the Boss
Result: 1.6 milion damage with a burst of 200k for a DK
I recommend using Dancing Rune Weapon because it mirrors your damage so it mirrors the Channel Flames DoT too. As a side note when you do it on CM mode just keep 1 Ogre alive CC'ed and use Dark Simulacrum on him on cooldown. This way the boss will be down in no time.
Notes:
This was done on CM mode, I'm not sure about the other difficulties hence the fact I haven't tested it yet but it should work just fine.
You may want to try this in a more trusted group than LFG.
No screenshot sadly because I was on CM mode and I was timed.
EDIT:I'm sorry if this is a repost but I've already searched and nothing popped out.