In my opinion both answers above shows only places where you can find all type of Dworcs, but ammount of Voodoomasters is quite poor.
Running around whole Dworcs cave is wasting of your time and making this bestiary harder, so I am definitely against to advice you - 1 level. You probably noticed on the map or maybe you know this place and know, that there are 3 ways to go on lower levels. Every way leads you to different caves. One cave where most of Dworcs are Venomsnipers, other cave where you can find a lot of Fleshunters and the last cave, where dominates Dworcs Voodoomasters. Entrance to this place you can se on the map:
Here you will find - 2 level and -3 level full of Dworcs Voodoomasters and it should be absolutely enough to finish your bestiary task faster.