I've had a lot of luck using a bench vice. Put a ~10mm box end wrench around the fat end of the cotter and clamp the thing in the vice. If you do it right, the vice will squeeze the cotter loose with a very loud bang. Make sure the wrench (or whatever you use) is thick enough to leave some space for the fat end of the cotter to move into otherwise you will just be pinching the cotter, which does nothing.

Hammering with or without the nut has always just ****ed up the cotter for me. Vice all the way.