Try this build if you are level 17http://us.battle.net/d3/en/calculator/witch-doctor#acdZX!Y!aaaaa
(feel free to move the skills around, that's just my configuration)
Grasp a group, run in and Soul Harvest, if you can't run out safely, Horrify.
Proceed to Spam [Dire Bats], picking up health globes that drop nearby.
If you run out of mana and anything is not dead, Poison Dart it to death.
Use Grasp again if a second pack tries to come in while you are still dealing with the first.
Horrify whenever you get swarmed and it is up.
Try and pick up Health globes every 5-8 seconds or so, to keep Gruesome Feast up.
At level 19 you will get Gargles the Tank, and this will only become easier to pull off. And keep in mind, it is already pretty easy. You should be able to almost one/two shot normal mobs with [Dire Bats], even without Soul Harvest stacks.