So far as I've seen, killing mobs is the only way to earn credit for this quest aside from destroying a demonic portal (shaped like four spikes meeting in the middle.) But there's lots of handy lootables that give you buffs like:
Supply Crate, which gives you bonus shadow damage,
Filled Draenic Flask, which gives you an extra 15% haste and movement speed,
Fruit Basket, which gives you an HP regen of 1% every 3 seconds,
and Draenic Gems, which gives you a bonus multistrike chance of 20%.
Some findings:
- [npc=84888] gives 1% credit.
- [npc=81605] gives 1% credit.
- [npc=85168] gives 1% credit.
- [npc=86024] gives 1,5% credit. (killed four of those, two gave me 1%, the other two gave me 2%)
- [npc=84945] gives 3% credit.
- [npc=86364] gives 3% credit (2% for the Binder, 1% for the demon, thanks mahonisaya!)
- [npc=84908] gives 5% credit (thanks Goddessa39!)
- [npc=81543] gives 7% credit.
- Destroying a portal gives 5% credit.
Will update with additional info the next time I pick this daily up! :)