I'm sure that you realize that the area you've described, being just the area you can see, is the tip of the iceberg. The bulk of the beast is below grade, and as you expose more of it to bring it down below grade, it will probably get bigger...a lot bigger.
Before you start testing the waters of rock removal technology, you might want to expose the rock down to the level you wish to see it at, and then measure the area. Guaranteed it will surprise you how big it might get.
The methods and effort you might be looking at to remove the rock will depend on what type of rock it is. If it's granite, you're looking at quite a project as you will either have to use power tools or feathers and wedges in holes you'll need to drill. If it's a softer stone, you may be able to simply wack away at it with the sledge hammer and break it, piece by piece.