Here is the full list of all 8 JUST a Game achievements.
Kill 50 Balls in one game.
Kill 100 Balls in one game.
Kill 200 Balls in one game.
Kill 2 balls With 2 Bullet.
Kill 3 balls With 3 Bullet.
Destroy 5 waves.
Destroy 10 waves.
Destroy 15 waves.