Basement bathroom

Hi folks,
My abs shower drain pipe extends around 12" through the floor (concrete). Shower pan/drain requires that I cut it down below the concrete grade to allow room for the drain. I've chipped out the concrete a few inches around the pipe, what is the best way to cut it down? I'm thinking sawzall near grade then Dremel inside with cutoff? Let me know if this works. I also hear talk of a string saw but I'm not sure that would work here (below grade a few inches)

They make an internal pipe saw just for this. http://www.reedmfgco.com/index.html?screen=internal_pipe_cutters

available at big box stores??

You should be able to find an internal pipe cutter at the Big Box. That will work the best for your plan.