Your best bet would be a masonary cut-off saw, preferrably with a diamond blade for a cleaner, nicer cut. This will get you your main cuts, however, it will not get the corners (round blade, square corner). For this, the proper way to do it would be to use, what looks like a chainsaw, but has diamond tipped teeth on it, to cut out the corners, nice and straight. The cut-off saw you could rent from just about any equipment rental place. The Diamond toothed chainsaw is more of a specialty item though, and you'd probably have to call in a a contractor to do that. Best bet would be to have a pro. do it from the get-go.
Another way I can think of would be to get out your handheld demo hammer and chip away what's left of the corners. This would suck though. It wouldn't be as clean and would take forever.
You also might consult with Sam on here, as he is in the concrete business, as well as a few other on here. Also, IIRC, Wnaplay works for a concrete cutting company, so he might be the man for the job. I work at a equipment and tool rental place, and this is just the best way that comes to my mind. Might not be the absolute best way though.