Hello guys,
I am writing this guide, because I'd like to share my experience with you.
First of all, I am currently playing with a wizard and my stats are the following:
DMG:270k fully self-buffed
Toughness:770k fully self-buffed
Well my foundings are these, I have played Act I , second quest, second sub-quest "Explore the cellar". After you choose this quest you teleport to "The old ruins" and you go east up the road. You will find an area, which is filled with zombie creeps, which you nuke to get the EXP. Sometimes this area is crowded and sometimes not so much, but it is still pretty rewarding. After you nuke all of the mobs, recall and reset. And repeat.
For one hour, non-stop farming these mobs, I earned 580 milion experience. But you also have to take into account the performance of your computer, the faster it loads the more EXP you get. So roughly 600 exp/hour.
I hope I helped guys.
Good luck!