- Port to Garrison using your Garrison hearthstone.
- Talk to Seer Kazal.
- Chose the "Bastion Of Shadows" option.
- Enter the raid instance and immediately teleport out of the dungeon.
- Go somewhere you want to kill people eg Auction House.
- Port back into Bastion Of Shadows.
- Aggro the trash in front of Shadow-Lord Iskar.
- Wait till the "Fel Raven" casts "Phantasmal Nova" on you.
- Get out of combat and leave the raid instance.
- Return to wherever you came from and wait for the Phantasmal Nova to expire.
- Watch players die.
For most classes I have not found a way to leave combat other than clicking on "leave instance group", which works but means you will have to return to the garrison and Seer Kazal to set the process up again.
Hunters can use feign death instead and then use "teleport out of dungeon". Best way is to use feign death before your pet engages the trash, then teleport out when the pet gets the debuff. This way allows you to use the nova repeatedly without having to return to the garrison.