There's a ledge above the entrance [after the last boss you can drop down from it to exit the instance faster] that you have to be in LoS of.
After the group's dead the healer needs to spirit release in the instance [this is accomplished by starting the logout sequence and clicking the "release spirit" button as it's about to tick to 0].
After that the healer simply runs through the instance up onto the ledge and res's the group, now you can kill the black stalker without going through the rest of the instance.
It may take a few tries to get the release to work properly but it's totally worth it imho.
