This is my first leveling guide, so don't bite my head off if it sucks >_<
IDEAL PARTY
Shaman (19-24)
Druid(19-25)
Warlock(17-25)
Hunter(20-25)
Warrior(30-40)
I used a guildie for the warrior. I managed to level up almost 3 times in the same group.
Now, first you'll want to just cruise to the instance entrance, of course getting the warrior slaughter everything while casting the occasional heal and support spell. Make sure the Shammy is SSed. Now, when you get to the instance, you'll want to do the FAnglords in this order:
Anacondras
Pythas
Cobrahn
Serpentis
Now, the reason you want to skip the second one is that you actually obtain more exp from killing Pythas first, since by the time you start running to the second Fanglord the enemies should have respawned. Again, let the warrior kill everything, and between Pythas and running back to the second one kill Skum. If you really want to, kill Kresh on your way to Pythas, but it isn't essential. After killing Pythas head back and kill Cobrahn, proceed to Serpentis and kill the Verdin something-or-other. Before you head out, escort the Disciple of Naralex to Naralex' chamber, finish up with Devourer and such. You should either have leveled quite a bit (20-23 or 22-25) or gotten a lot of phat lewt. I leveled my druid from 20 to 22 in an easy hour with my xp bar to 23 about 70% full and obtained most of the Fang set (except the legs). It also works to wait till the warrior gets a boss low, get someone to boot you out of the group, kill the boss, and quickly get invited back before being evilly hearth by Blizzard's "no group no game" gig.