I've fiddled about with stuck cotters for the last 50 years, and there is no truly foolproof way, but the way that always works for me is to:

1. Remove the mashed, bent, and otherwise unusable threaded end of the cotter. I typically give it a couple swacks with a punch and it breaks off.

2. Take a Dremel and grind a small divot into the approximate center of the thread-end of the cotter.

3. Drill a 3/16' hole about 1/2" deep into the cotter.

4. Punch it out with a 3/16" punch. Use a brass hammer. Always.

This method takes some time, but it has never failed me.