I usually hit Marleybone around level 27 or so. I have very few problems. Marleybone is about patience. You need to learn that for the upcoming worlds. Take your time, get your strongest gear and you'll do fine.
Hmm, My death wizard got there at around 27-33 so I say those numbers. But my life go there at Lvl 22 (Autumn Pixiedust), and she got hit in the neck hard! (that means that she gets defeated easily)
It depends how good and what lvl you are on it really just depends if you are high good for you but if you are low try to make put it as a goal. just like me.