Cut it with a chainsaw, leaving an extra quarter to half inch or so.
Clean up the extra as follows:
Put your router on a "sled" (i.e. bolt a long board to the base, with
a hole for a straight bit). Put a straight bit in, set it 1/2" or so
lower than the board.
Ok, now clamp boards to the top and bottom of your slab, such that the
edges are 1/2" beyond where you want the final edge to be.
Put cleats on the bottom of the router's board such that the cutting
bit just reaches to these new boards. The cleats will hit the outside
faces of the new boards just as the bit hits the inside faces of the
You can now freehand the router between the two boards to flatten and
smooth the rough ends. You might want to start with the bit only 1/8"
exposed, then 1/4, then 3/8, etc.