if you are only clearing gr28 with a tiki-lock doc then it is gear that is the issue or skills (fyi if you are using the 0dog sac version then you die a LOT with botting it doesn't seem to do sac-doc very well)
personally I use http://us.battle.net/d3/en/calculator/witch-doctor#WlXQPY!TWVe!bbbbbc for botting with SMK + US mojo, 3p blackthorne, 2p aughild, tiki mask, T&Ts, rorg, unity & cold immunity amulet, bot is happy to do 35-39 seems to really struggle at 40+ though but then it is RNG