There's a relatively quick way to level up. You'll need to have Hordes of Underdark chapter 2 unlocked.
Once you get started, leave the main building and enter the building directly to the south. There you'll find a NPC called Zesyyr Maevir. She'll give you a quest to assassinate her mother. Accept the quest and leave the building. Head straight north and you'll come across a group of nobles. Click through the conversation for a quick 500 XP. Continue north, slightly west, and you'll find some NPC and a drunken NPC. You need to pass a, not very strict, persuasion check and you'll get another 500 XP. There's also a +2 alignment shift towards good, so skip it if you're evil. To the right of the drunken NPC you'll find the building guarded by two guards. Talk to them and they'll take you inside where you'll have to fight 4 NPC's. Finish the fight and you'll get 5000 XP. All in all it took me about 3-5 minutes per run for 6000 XP. Export your character and repeat.
Also I was able to complete the fight with a crappy geared level 15 sorcerer, but barely.