This is done on the level Genosha. At the beginning of the level, it will start with you having Wolverine over one of your shoulders, with a cut-scene which will only be a few seconds.

You will throw Wolverine on the floor and he will be unconscious. Next you will go over to him and have the option to 'Bitch Slap' him using X, or have the option to press B and be a quitter.

Obviously, you will be using the first option for this one, just make sure you continually slap him, and keep in mind this will take about 100 slaps to unlock, but rest assured, it will come eventually.