This is the "gimme" achievement of the game.
Once you start the game, choose a character, and you'd go into your first level. By default, this is a "Smack Attack" level that requires you to land 300 smacks on your opponents. Set the game's difficulty to "Easy", and start it.
In "Smack Attack", you'd need to basically attack the AI with weapon pick-ups. Make a run for a projectile type (stones, for example), and start spamming
to shoot at your enemies. Each time you land a hit, you get 1 point. If you happen to completely destroy an enemy, you get 10 points.
Simply keep shooting, and killing enemies, until you land 300 smacks to win the level for the achievement.