Say what you will, but the Rogue/Druid combo in Oasis is the best team for rapidly gaining levels.
The target: Crocs (all of them... blues to reds)
The Druid's Role: Snare and Fear Animal.
The Rogue's Role: Backstab, backstab, backstab.
Typical encounter: Druid snares croc. Croc comes after druid. Just before the croc gets within snapping range, the druid fears it. Enter, the rogue. The croc runs directly away from the druid, and the rogue follows on its tail attacking relentlessly and BSing every 8 seconds. The fear will wear off, and the druid reapplies when he sees the croc turn on the rogue. The rogue may get hit 5 or 6 times during this entire process. The druid will usually have to cast fear 2-3 times... medding the rest of the time... and having more than enough mana to heal up the rogue at the end of combat.
Using this method, the druid will stay nearly full of mana, and the rogue will keep at least 80% of his/her health at all times.
You could substitue the rogue for some other melee class... but, in this scenario, who else deals more damage??? When Oasis is uncrowded, you can pull/kill crocs at a rate of one per minute.
This is the ultimate LOW RISK - HIGH RETURN combat in the game. And, it's a highly viable tactic from 14th to 20th. The only drawback is boredom and total lack of loot. But, if you're looking to level through the teens as fast as possible, this is it. Good luck.