The Witch Doctor is a very diverse character. It is a spell caster that uses voodoo magic to cause pain and rain hell on their enemies.
They are not necessarily like the Necromancer class but they do rely on a lot of poisons and damage dealing abilities that are similar.
One of the ways to kill mobs quickly is to learn how to hit and run. They have skills like poison dart, horrify and acid clouds which deal damage over time. This means that your opponents are not killed instantly and it is important to keep them away from you while the poisons and damage take its course.
Mind control is a very good spell for crowd control as it turns enemies upon each other and they make it much easier for you to pick them off.
Be sure to buff up some defensive equipment on your character because it is very important to survive. You are not like a Barbarian or a Monk. The character is more frail and you need to employ a lot of strategy especially when you are climbing up the higher levels.
Learn to hide behind your minions like the Fetish army or Zombie followers. Like its predecessor the Necromancer, summoning your monsters to do your bidding is winning half the battle already while you blast away at your enemies with your spells.
You can even summon your own zombie dogs and get them to explode at your enemies. Timing is really important.
Learn to stay alive using soul harvest. Keeping yourself alive using these means will help you to survive longer in battle without running back and forth to down to reload on items.
Remember not to stick in one area and grind monsters for too long. You might be tempted to do so, but remember that there will always be better rewards if you venture out.