This achievement requires you to get 100 smackdown attacks.
A smackdown attack is when juggle the enemy in the air and jump so that your higher the enemy and attack so they hit the ground hard for extra damage and occasionaly a few low level item drops. This can be done with either handguns or machine guns. I vaguely remember doing it once with a grenade but cannot confirm this.
So to land smackdown attacks, first it must be during a hero action while running on the ground and an enemy is knocked into the air. While still running you have 2 options. Bonus shots or smackdown attacks. We want smackdowns so jump your character (X while running) and when your higher than the enemy in the air release your charged attack. They will smack down onto the ground and bounce back up. Depending on how much of the hero gauge you have left you can repeatedly smackdown 2-3 times on average.
Best place i found to to do this was the first fight in the arena. I did it with a machine gunner cause you cant kill only 'scratch them' but either works. So just run back and forth plotting hero actions and knocking them into the air and back down until you get it.
This is actually an easy achievement, i had 74 by chapter 7 before i decided i wanted the achievement NOW (lol) By playing through the game you will surely nab this and several others. Remember if you don't know how many you have the Game Records board inside the arena keep a track of this and several other stats for achievements.