Post Mercenary Leveling - Blightfire Moors
If you're in the low teens when entering Blightfire Moors you'll want to stick around the PoK book in the western part of the zone, also near the Crescent Reach zone line. Kill rats and snakes here until you can handle the Gnolls to the south. The Gnolls are a bit more annoying than the rats/snakes since they have a tendency to run.
Once you reach 22 or 23 you'll be the right level for the next location which is a large field filled with bees. If you've got a Gold tank mercenary he'll be able to handle one of these at a time and they'll net you a ton of EXP for this level range.
I usually stick to the northern part of the bee area. There are a lot of single pulls up here and it's safe to AFK up against the wall if you need to use the restroom or something. The bees here are great EXP until 30 and then they start to slow down a bit.
Once you reach level 30 I recommend you travel north to Goru'kar Mesa where you'll want to kill bears and wolves until you reach level 35.
Blightfire Moors Original Almar's EQ Leveling Guide
This guide may contain errors or inaccuracies since the game has changed a lot since I wrote it.
It also contains a lot of information not found on this page about the zone and the mobs in it.