this game is pretty easy... when you got enough cash... try buying the rocket launcher and so its upgrades and its ultima.... now when its already a spread type rocket launcher... try using it at point blank range w/ the zombie and you'll see that the shrapnel that was released will almost takes the rest of the lesser zombies...
in lvl 25... just focus on putting the goddamn ball to the center of the landing pad 'coz physics doesn't apply here, meaning if the ball landed in the center of that pad, the ball will just go up and you can control the ball easily hence, upon doing this, there is a chance that the ball will just bounce up and down (a bit of luck really) XD