Depending on the size of the brush, you might be able to pull some railroad irons over it to break it up.
That's just some old railroad irons with several eyelets welded along the top edge and then heavy duty steel cables running from the eyelets to a clevis such that you attach to the drawbar on a tractor.
If the brush is very big, this probably wouldn't work. If I was to do it, I would probably want to do it on a really cod day, especially if it has been below freezing for several days, in hopes that the wood will be brittle and break off easier. Then a few months later, if the brush is dead, do it again or use a set of nobles to break it up better.
For what it's worth, when I was a kid we used three or four railroad irons in a row to smooth out the land before planting so that when we watered it (flood irrigation then), there wouldn't be many high spots that wouldn't get water.