Up in the Eig Mountains, at the whitehand camp. Theres a boss called Diabolist Aino, she's 54. She will spawn 4 "Minions," but they are not labeled minions and you receive full exp from them. Anyways, don't attack the boss, let her live, your group will out heal her damage. Every time you kill the 4mobs she spawns, they respawn and you get endless waves of exp. I did this for over 40mins, got a whole level at 55 in less than an hour. With a lvl 55 conq, 44(apprecntice to 54)HoX, 55 Bear Shaman.
Coords are @ 945,764, i posted a map. I have included a screen shot of the exploit linked below.