Quickest Way 1-50

First, it is important for you to know how to reset an instance without logging out of the game. Hit "P" to turn Auto-Party on. You should have a little group icon in the top right corner of your screen. Now when you enter an instance, you should get a message that says "You are now the party leader." When you are done in that instance, type /leave and you will be teleported back to the station. Enter the instance again, and it will be reset. This is very helpful for farming Shulgoth and Sydonai.

Onto the Guide!

In normal mode the level progression follows the quest progression. You can just do the quests until you finish Normal mode. After you finish Normal, you should be around level 30. If you aren't 32 yet, grind in Hell until you get to 32. This should not take very long. It was about 2.5 bars per run when I did it. If you get bored with Hell, you can also run the low level quests and instances in Nightmare, but don't progress farther than Shulgoth.

After you hit level 32, start killing Shulgoth. When you do this, you should clear the entire level, not just run straight to Shulgoth. After you clear the instance and kill Shulgoth /leave to reset the instance and go kill him again. You will get 500 Ability Points for killing Shulgoth 100 times. This is the perfect opportunity for you to get those 100 kills. You will also get a ton of good loot so you can get geared up. If you kill him 100 times, and clear the level each time, you should be around level 38 when you are done.

After your 100 Shulgoth kills, proceed down the quest line until you get to Charing Cross Station where you can run Piccadilly Approach. Go in Piccadilly Approach and clear the instance, and continue on to Piccadilly Circus. Clear Piccadilly Circus until you find the glowing hellgate. Go in the hellgate and clear it, and then go into the rift called "A Step Into Hell." Clear that instance, /leave, and start over. Do this until you are level 40. It is around 2 bars per Piccadilly run.

After you reach level 40, proceed down the quest line until you get to The Mind of 314. Clear The Mind of 314 instance, which is full of level 45 mobs that are fairly squishy. When you find Luscius, talk to him and he will spawn the big baddies. These guys are level 47. Kill them if you can, but if you can't or it takes too long don't worry about it, just don't talk to Luscius again until you can. Abandon the 314 quest and the game will kick you out. Log back in, get the quest again, and clear The Mind of 314. This is a fast run that gives a little under a bar per run. Start killing the big baddies as soon as you feel it doesn't take too long, and you will also get some decent loot.

Doing xp runs from Temple Station to Charing Cross Station is also good xp from level 40-50. As you level up passed 45'ish, do some quests and some Temple Station runs to break up the monotony. Go explore areas as you can get to them, but get that grinding in when you can. Keep at it. You'll be 50 before ya know it. :)