1) Soak manifold to downpipe nuts with PB Blaster or Kroil for a day before doing the work.
2) Use a long extension and loosen in a series of 8-10 thread attempts.
3) If/When the nut tightenms up on the stud take a break and use a propane torch to heat the nut only. Then try another 8-10 thread attempt.
4) Do it on a rotation, not just one at a time.
5) Cut bolts that hold Cat to Downpipe to take weight/torque stress off of the manifold to downpipe connection. This will prevent you from snapping off one of the studs if the cat assy moves during removal.