Questing on my alt I found these buff items and researched them on wowhead, they do stack with elixirs so they say I havent tested it myself yet.

Scourgebane Draught and Scourgebane Infusion.
The draught gives 30 ap vs undead and the Inusion is 15 sp vs undead. Not sure if it would benefit healers?

This could be useful against HLK.

they are rewarded for a repeatable quest to collect rotting hearts or spinal dust in the ghostlands.
turn in vendor is located in Tranquillen. The level 17-18 zombies/skeletons drop them fairly good, but to get a stack of 20 i bet youd have to kill 250-300 of these mobs as it takes 10 of each item to get 1.

not sure if it would be worth it, for the min/maxer who cares about every 2 attack power for sure. but since they last 30 minutes you'd need 2 stacks per week of attempts on HLK, unless of course we kill him i bet looking at an hour or two a week of farming per person just to get these 2 stacks.

if i get time today ill try to farm up a stack for crit and see if its noticeable.