I have a been trying to solo farm Iskatu for experience to get my levels up to a point where playing Hell is less "squishy". For some reason I am not finding the right build for the Witch Doctor to make killing him alone on Nightmare with the templar effective. 

I have been using this build which is nice in doing passive damage but is not effective on the higher levels. 
<http://us.battle.net/d3/en/calculator/witch-doctor#aZUdjT!ZVW!aYZaYZ>

What is the most effective build for killing this boss that places an emphasis on speed to complete specifically for the Witch Doctor? How does this build change when you fight him on Hell vs Nightmare.