It may be a repost due to it's widely known for now however I didn't find any "Nemesis" topic with the related trick, only some replies with inaccurate / missing information so this may still be helpful for some people I hope.
It's already known (maybe intended too) that you can get 5 kills for a single kill if all your party members are on the same quest and are in range when the killshot happens.
1. Look for / hire a rezzer class and a lvl 100 characher of the race you want to kill (in your opposite faction)
2. Pay them for a half-hour job if you need to.
3. Look for people with the same quest (e.g. LFG tools), party up 5 man.
3. Go to an unpopulated area.
4. Start killing the guy, let the rezzer chain-rez him.
5. In about 20 min the quest should be done. Move to the next one.
Notes:
- The time it takes may be longer if people finish their quests earlier than you - in that case you should invite another one, because
- People who are finished (reached 500/500 kills) do not count anymore toward your kill numbers. They can leave.
- No, you don't need to wait the 2 min rez time, the criteria is: kills must be honorable kills and a rezzed character fulfills this criteria. You can chain-kill him which is the lifeblood of this method.
- On PvP realms this can be easily ended up a massacre if you don't do it in a dead place.
- You can hire people from trade channel for low amount of money, we paid only 500g to our Pandaren dummy.
https://i.imgur.com/WvBS4vO.png
https://i.imgur.com/PB1ymJP.png
https://i.imgur.com/vq1hx2B.png
On the first glance I was unsure where to post this, Exploits or Guides. However I don't really think Blizzard wants us to do these long quests in this accelerated way so I posted it just here. Feel free to move to guides if you feel this doesn't fulfill the exploit category.
No, I can't see Elite section so don't call repost pointing there.