Well, the concrete is only a small part of the work. It's all the dirt you may need to move under the concrete that gets tiring. And then, to do it right you need a gravel bed before the new line install, and then some more gravel and a whole bunch of dirt to fill in the trench.
You haven't given much details to work with. Commercial building? Old residence of some kind? Are there many partition walls in the building? Can you open one end of the building and get a small backhoe inside? Then again, can you put the new line outside along the building - only going under the slab where you need to, so that the slab is compromised as little as possible?
Every building is different - and each may have a different best solution.
The common way to make sewer repairs under a slab is to try and map the lines - and then cut paths in the concrete over the top of them - a cut 8 to 12 inches each side of the pipe. The width of the "path" will depend on the line size and how the dirt is removed. Then the paths are broken up by jack hammer or hand sledge and the concrete debris removed. Now comes the fun part - shovel work, more often than not. The last one we did was an an old apartment house and the concrete floor covering all the sewer lines was in the basement. Lots and lots of shovel and wheel-barrow work - running the wheel-barrow up a ramp on the stairs. The worst part was the old lines that had to be removed were buried in wet gumbo. Arggh! What a pain.
Hard to tell you much about your situation without more details on the building.