This power leveling strategy will get you a full level in about 5-10 minutes (1/20th of a level in 20 seconds at best). The problem is that you have to do the same thing over and over again, but it works until level 60. This is not my original strategy, it was posted on a few other websites and there is a nice video of it on YouTube (linked below) but here is how it works:
You can start by creating any character you want and having a friend help you reach Act III. This can be done by joining a friend’s game who is at the last quest of the Act and then joining them for the boss kill. There are no Act level requirements, but you won’t be able to progress to the next difficulty until you unlock it yourself and reach the minimum level (25 for Nightmare, 50 for Hell).
Once you reach Act III begin the first quest and have your helper get you to the Skycrow Battlements (seen in the video below at 1:38 through). Bring a follower and equip some sort of snare ability (Demon Hunter: Caltrops, Wizard: Frost Nova, etc). The enemies coming down the stairs will bunch up at the corner where you can lay a snare and watch them die to the catapult attacks from above. This will make killing possible without good gear or assistance. You will get the hang of timing the snares right for the catapult attack.
You can level all the way to level 36 this way and then you’ll stop receiving experience, but there is no reason to stay in Normal difficulty once you reach level 25. You can get to level 52 in Nightmare, but you’ll start going really slow from levels 43-47 and eventually gain no experience at level 52. You can, in theory, get all the way to 60 doing this in Hell difficulty.
Here is a video demonstration of this ability.
This reportedly will get you to level 50 in 4 hours, beyond that I’m not sure since I didn’t test it that much. If you want to get every character class to level 50-60 you can do it this way before it gets hotfixed. Chances are it won’t last long and as a warning: you might get your character rolled back or account suspended temporarily (I doubt it though).
Let me know if you utilize this exploit!
Important: Many of the strategies and tips I share here were originally inspired by the people at the Diablo 3 Gold Secrets Forum. If you're serious about making the big bucks in Diablo 3, or just being the best players, you need to signup ASAP.