Because of some posts I got below I want to add something before you start reading the guide. I would like everyone to understand that this guide will NOT make your leveling easy. This guide will make your leveling unbelievably fast for solo, but you will have to work for your levels. The high experience comes at a price. This guide is not for people who expect to hold down right mouse button and win the game, and when that fails blame others for their unsuccess. This guide is not for people who are stuck on lvl 35 and can't get any further. This guide is not for those of you who are asking in general for help on leoric on hell. My guide will not make you a good player, even worse - it requires that you already are one. The mobs hit like trains and they don't die easy, but that doesn't change the fact this is probably one of the fastest ways to solo level. If that didn't discourage you but got you intrigued - please keep on reading.
Ok guys let me teach you how to level your chars insanely fast. I'll keep it short and simple. The place is the new secret level - Whimsyshire. It takes a little investement - you need to buy a weapon and preferably some armor. Ok more detailed now.
I like to upgrade my gear every 10-15 levels and everytime I change difficulty. l go on the AH and I buy new gear for every slot, I pay 2-3k an item, up to 10k if it's a weapon or something very, VERY good. I never waste more than 100k for all the slots combined. You get those money back while leveling there of course. Is there a trick ? No trick, you go in the secret level and kill everything. Why am I even posting this common knowledge ? Because it ain't knowledge and it definitely ain't common. Let me run you some numbers. I wrote some stuff down every since I entered hell. Those numbers apply for hell, on the lower difficulties it's faster and easier.
Average run length FULL CLEAR - 13 minutes no rushing (This time includes opening all the clouds, presents, mushrooms, etc. This also means I have killed all the mobs and checked the entire map)
Average experience gain for the already mentioned full clear - 8 bars (could get up to a level depending no how lucky u are with the elites, lower I had is 7)
Is it hard ? As soon as I hit 50 I entered the secret level on hell, cleared it 4 times so far without a problem. It is easier on the lower difficulties. (I did struggle a little bit on lvl 25 on nightmare but that was because I had 0 vit on my gear)
AND NOW - THE CROWN JEWEL
This math is done as a barbarian! From what I collected so far demon hunters and wizards are doing it pretty much twice as fast. These 2 + monk all die less than a barb. As a barbarian I die every now and then for obvious reasons and I also run slowly because I have no mobility spells on my bar other than a single charge. Not even leap. So basically up there I gave you the worst case scenario - a barbarian.
When to enter ? Don't remember when I entered normal ones but something like lvl 10ish should be ok, maybe even lower if u get good weap and some blue items. Nightmare on 25, if u're barb u'll need some vitality gear. Hell on 50, entered it without a problem after 60-70k spent on items. Good weapon is always a must. I am kinda tired right now from a long day and I tried to keep it short even though I can see I didn't do a very good job. Either way if I missed anything and you don't understand the guide completely - feel free to ask me questions.
Forgot to mention and this is important - always buy helm with socket and put the highest red gem you can get in it.
MONK UPDATE:
Since I haven't leveled a monk at all yet I decided it's time to try one. At lvl 15 I entered normal whimsyshire and so far it's by far the easiest class to kill the mobs with. I can probably clear it with 4 people in the group, although I haven't tested that. I entered the place as soon as I hit 15, I hit 18 with the last pack I killed. So basically it gave me 3 levels. How long it took ? Well I didn't do it as a speed run, I went there to see if it's possible, and me and my friends were busy laughing at how easy it actually is. If I am focused to clear it as fast as possible it will take ~15-20. I also took some screenshots because I know a lot of noobs will come to tell me this is not possible. If anyone comes telling me how this is a lie I'll prove to him that he's a noob with the screenshots. I won't upload them here because there's no point to flood this with 10+ pics. I'll update it again when I get in nightmare on 25.
MONK NIGHTMARE UPDATE!
I said I will post an update on nightmare monk as soon as I hit 25. Since the normal mobs were too easy for me I decided to go in nightmare on lvl 25 with my lvl 15 gear and stats. The white mobs were dying easy but the elites were giving me a hard time. Doable but slow, which beats the idea of being there in the first place. So I decided it's time to go shop for nightmare. Costed me about 150k to remove all my lvl 15 items with gear appropriate for my level. What I can say after doing so is that the monk is by far the easiest class to level. White mobs are not touching me, the elites are doing damage to me but I can outheal it without much problem and proper skill usage (after all skills are there to be used) The experience is insane, I'll check how much XP I get when I finish the run. I entered as soon as I hit 25, after a full run I am halfway to 29.
Also at this moment I want to send my regards to reptyl, the first post under my guide, saying Nightmare at 25 is imposible. Actually he said it's impossible at 31. Well it's doable with lvl 15 gear on lvl 25, and with lvl 25 gear it's just easy. Too easy I may add.