Hi.
I found this mob with 70k HP and it rewards quite a lot of XP when killed.
Mob name name is Giant-Slayer Kul
So what you do is:
1.) kill Kul
2.) join one of the millions of autoinvite groups to switch realm.
3.) repeat step1
I killed him with +45% xp modifier from heirlooms and +100% from beeing rested.
at level 96 this was 5922 XP per kill
at level 91 it was around 17k xp per kill
at level 92 it was around 14k xp per kill
There are a few more XP modifiers that i forgot to use, but i will mention these here.
With these buffs you will gain even more XP per kill:
Elixir of the Rapid Mind - Item - World of Warcraft
Excess Potion of Accelerated Learning - Item - World of Warcraft
Darkmoon EXP Buff