For the life of me I can’t get one made to work, so i ended up resorting to an ingame macro. This works flawlessly at level 28. A lil mana intensive, but it works for dungeons, questing and pvp.
Hope it helps some one!
/assistfocus [exists,nodead]
/targetenemy [noharm]
/castsequence [nochanneling] reset=target Shadow Word: Pain,Devouring Plague,Mind Blast,Mind Flay,Mind Flay,Mind Flay
/cast Power Word: Shield