Dead Rising 2

To unlock the Smash skill, you will first need to level up to around 35-45 first. Once you see "New Skill available" or "New Skill Unlocked" after leveling up, you can check to see if it is the Smash skill.

To check and see if you have the Smash skill unlocked, press to pause your game and go to Status. Press to go to Skill. If you see Smash on that list, you have it. If you don't, you will have to level up some more until you unlock it.

Once you have the Smash skill, all you need to do now is kill 100 zombies with it. Make sure you have 1 inventory slot open (not required but you will drop whatever is in your equipped slot), and press to put away your equipped item. You should now just have your fists out. Now find some zombies and use your Smash skill on them until you have killed 100 zombies.

To use your Smash skill, simply press and hold + near a zombie to grab them and smash them to the ground. It helps to have an object such as another zombie, wall, ect in front of the zombie so when you smash them into it, it will be an instant kill. If you just throw the zombie on the ground, you will have to perform the smash skill again to kill them. You will know if you kill a zombie as you will be rewarded some PP.

The best place to grind this is obviously outside. Though smashing them on stairs and such also worked out quite nicely as you just smash them into the stairs.