Page currently says "Warning: Killing the right-most Giant Skeleton Hellhound will prematurely enter the second wave."
My memory was that the cut scene triggers when you have killed any 3 of the 4 Hellhounds. I've just done a run and killed them right to left, each with a single shot of Firebolt. It triggered on the 3rd kill, so I didn't get to kill the left most Hellhound. Can someone else verify?
I think the advice we should be giving here is first kill any Hellhounds that are separated from the pack, in the hope that you can kill the last two with a single AOE skill, such as Fireball.