Go to the local Rant-All. Tell them what you're trying to accomplish. They should set you up with a LARGE hammer-drill with bit...likely a star bit.
You should be able to pick out a grain line in the boulder. Drill a series of holes in a line, spaced about 8-12 inches apart and about 3" deep.
Then, insert a set of feathers in each hole and "tap" them in until the boulder splits. Obviously, you'll want to split the boulder into the largest sections you can, though keeping the sections small enough to be manageable.
The best part is you get to repeat this, over and over and over and...
The key is the hammer-drill and bit. Rent the largest drill you can. It'll be worth the extra dollars.