I had my g/f at the time hold the wrench inside the bus while i crawled underneath with the rattlegun (impact wrench) The wrench on top just needed to be held at the proper angle, then it would spin, hit the seat post and not require any muscle to keep it in place. I was able to break one or two bolts starting off, but i also stripped a couple of the bolts. The angle the wrench on top was held wasn't ideal, and allowed the head of the bolt to get stripped.
After 2 or 3 seats, i gave up on this idea and fired up the angle grinder. I ground the heads off of the remainder of the bolts. It was late december in michigan, and crawling under the bus with my carhats on just wasn't much fun.