You're 100% right about only having one shot with the saw!
Gary G's suggestion is a good idea. When I did something similar for my SIL and her husband I made three jigs but did not connect them. I just measured very carefully to make sure I was cutting on the proper side of the line and not into something I wanted to keep. I then made everyone involved verify that I had set the jig up right because I almost cut on the wrong side once.
My original plan was to make small boxes of plywood and lay the entertainment center face down on them. I wanted to make them tall enough that my circular saw would fit underneath at the start of the cut . I didn't have enough plywood so I ended making several "T" instead. I laid on the floor so I could see the "T" so I had two points of contact with each T on the entertainment center. I hope that makes sense, if not, I'll take a picture of the "T" and post it later for you (if I figure that part out again).
I cut one side and then used that cut to as the starting point for the back, cut the back, and then used that as the starting point for the third side. You will need several clamps and I would suggest a couple of friends to help make sure that EC doesn't move too much on you when you make the third cut.
Hope this helps. I could go into more detail but this would turn into a novel. I'll try to clarify if what I said doesn't make sense.
And if I figure out my issues with posting pictures, I'll post a 'before' and 'after' of what I did.
Think I figured out the pictures...sorry that they are so big.
Turned into