Managed 4Horsemen 25 as Destro. Was a bitch to do and took several tries of my voidwalker dying just before the Thane so I then got slaughtered by melee and marks.
Odd observation - doomwalker would spawn, then immediately attack Lady Blaumeux. Whilst this did stop her from casting her thing, it took away needed DPS on the Thane and Baron. Any ideas how to stop this? Doesn't hugely matter, I got Damnation first try.