Depending upon the stubborness of things I have had some success with performing the following:

1) soak the area with a good industrial grade rust/corrosion solvent like CRC or similar, not WD40 or Liquid Wrench. I mean really soak it and resoak it.

2) Use a a protective mallet to tap the assy. here & there.

3) apply "Freeze Spray" which is avail at electronic supply outlets. If none is handy you can invert a can of Dust Off and that will get darn cold too. The electronic component cooler is even colder though. Spray until the bottom bracket is thick with frost. DO NOT spray the bracket shell.

4) tap with mallet again and now use the spanner tool or drift if necessary.

The metal should contract enough to loosen things, provided it wasn't cross-threaded to begin with!!