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.