We used a compound pipe wrench for years to loosen stuck pipes. It has a large mechanical advantage, and you don't destroy the pipes or fittings. A 90 degree pull on that wrench only turns the pipe about
1/10 as much.
If the fittings are cast iron, Use two 3# hammers to break the fittings. Use one to pound and the other as a backup. That won't work well for wrot fittings though. If you don't use the back up hammer, you may cause a leak at another joint. Use gloves and eye protection so you don't get hurt. One guy holds the back up hammer and another pounds the fitting from the other side.
Of course there is always a sawzall. But then you have to thread the pipe for your new connection and that takes a very square cut.
Stretch