After doing this myself, definitely go all affliction at the very minimum up to pandemic and instant howl of terror. Shadow on the priest with the mana efficiency talent and the 20% mana reduction SW:P glyph. Then, just run through fields of mobs using 1x SW:P plus 4x corruption on each mob, wait until you have 5-10 dotted, then use 2 aoe fears one after the other and watch as all 10 mobs die in about 12 seconds.