okay, i doubt this can be done by all classes, but on my hunter (Beastmaster spec) i tried solo some of The Blood Furnace in Outlands...
I Ice trapped 1 of the orcs, and then killed the other 1 np.. i did NOT have rest exp, and i got 2.5 K exp.. thats fkin much!
now i went outside instance, healed pet and drank some water, went in again, killed other 1, ran out, reset, etc.
i actually got ½ level VERY fast...
(and dont say this can only be done by 9/9 cryptstalker hunters, beacause my hunter is mainly green equiped, but still had no big problems)