DISCLAIMER: I figured this out myself with a friend, and I have never heard about anyone else ever doing this! We tried to look for similar mobs for this trick but had no luck.
NOTE: You need a laptop or second PC and a high level healer to do this. Running two WoW apps can get risky due to late heals.
I originally wanted to make a video of this (almost 2 years ago, lol) but never really got around to it so I'm just going to write a guide for it instead. This is done in Stranglethorn Vale, and can only be done with a Rogue or Druid (never tried it on a Warrior, but it might work too).
The key to this trick is the NPC Zanzil the Outcast.
Zanzil the Outcast - NPCs - World of Warcraft
What this NPC does is it spawns two totems.
The first totem spits lava, destroy that immediately as you see it (use a macro if you must to target it)
The second totem is where the trick lies! The second totem continuously summons undead skeletons which are relatively weak and give a good amount of experience!
This is how you do it.
1.) You pull Zanzil out to the coastline where your healer is standing and targeting you (I had the healer on my laptop)
2.) DO NOT ATTACK ZANZIL! Just kill the skeletons and lava totems.
3.) After a while, you start to get a feel on how many skeletons you can safely handle. This varies between characters depending on gear and level.
4.) Once you get the feel for your limit on skeletons, kill the skeleton spawning totem BEFORE YOU APPROACH THE LIMIT! He will summon another totem shortly so dont worry.
5.) Heal yourself using your healer standby account whenever necessary.
6.) NEVER over estimate, it's always good to play it safe. Dont leave the totem up for too long as too many skeletons = BAD.
PROTIP: I made macros to target the skeleton totem and the lava totem to target and destroy them quickly if I need to. It gets hectic with a lot of skeletons up!
NOTE: TOO MANY SKELETONS WILL KILL YOU! KILL ZANZIL IF IT GETS OUT OF CONTROL, HIS RESPAWN TIME IS PRETTY FAIR.
For maximum efficiency, you never want to NOT have skeletons up, so make sure you figure out your limit quickly and try to stick to it.
I did this with a rogue 2 years ago, and a druid a year and a half ago, Combat and Feral spec respectively. I was able to level from 40-51 in one day.