First you must have completed the EE.
Next, you must go down into the basement of the spawn cabin (Where you originally turned on the power), and to the left of the zombie dagger throwing mini-game, there is a rack, and you'll see the "Chainsaw" weapon the slasher uses...pick it up.
Now, when you meet the slasher while in Rave mode, wait for him to charge at you and dodge his charge, after he stops he'll be stunned for a brief moment which will allow you to run in and attack him with (LT) which will grind him and kill him very easily.
Note: You must have completed the EE during that playthrough. If you were to quit and then go back into the game later on, that's a new instance, and the EE would need to be completed again...