If your ground is too rocky for a tractor brush hog, then I'm not sure that a walk behind brush cutter with a metal blade is what you can use, either.
A gas powered trimmer with a metal brush blade is okay for limited areas, but not good for rock and not good for large areas.
I have used a walk behind trimmer with the flexible lines, and the brush worked those lines over pretty quickly. The one I used was not self propelled, either. A good shoulder trimmer is easier to use and at least you have the option of a metal cutting head if you need it.
Sometimes you can spray brush and at least see what you've got to deal with before you start cutting into it blindly.
A photo of your site might help.