There is no way for you to prevent the client side from being manipulated. It is being controlled by the player and he could introduce subtle changes to the client-side application logic that won't be detectable on the server side. The only solution I am aware of is sending all user actions to the server (all at once at the end of the game or continuously while the user is playing) and having the server verify them (recalculate the score). If the actions result in the score that the user claims to have achieved then accept the score. If the actions don't match the score - reject. It will be much harder to generate fake actions that are logically consistent. It won't prevent all cheating techniques however (google for "aiming proxy", something similar might be possible in your game as well).
