My witchdoctor has hit 50, and has never had a particularly difficult time... her secret though is that she always runs with a privateer, which may not be an option for you!

A witchdoctor should hang back and attack from a distance. Where possible, I prefer Brynn's "spells"/special attacks (which always hit) rather then her staffy weapon attack (which the target can dodge). I am ruthless about discarding (right-clicking on them) spells/abilities that come up that are not much use in the fight. Because a WD is not designed for melee, make your first mate and most-likely-to-show companions strong melee types. Unfortunately, most of the companions you receive "free" as a witchdoctor tend not to be the best melee types (the animals in particular are OK at first, but don't promote). If you can afford a few crowns, recruit Lucy Sterling. All of the starting companions and all of the companions the different classes receive at the start of the Gunn's treasure/Ratbeard quest are particularly powerful, since they start with more epic abilities than most companions and receive more as they promote up (and all of them will receive two promotions by the time they are epic). Of those ten companions, eight are melee types - they can be extremely useful to you. Musketeer and other witchdoctor companions are going to want to skulk in the back and lob damage from afar... that's your job!

Good luck!

-Michael