I find this out while lvling my druid today, there are 3 NPCs near the wall where Nak'Lak spawns in Valley of the Four Winds, they usually and intentionally attack the smaller mantid in the area, but I found out when they get hit by either Blade Flurry or Tornado from the boss they attack him with amazing damage and kill him in about 30 sec, this is not intended as they are only meant to kill the lesser mantid coming down from the wall, hope this helps some people.
NPCs: Fo Sho Knucklebumb, Ko-Yan Na and Jooru Elderblade