Most of the games have walkthroughs available that you can use to pass levels where you're stuck. Unfortunately, this game lacks the availability of such walkthroughs. You will have to pass the levels as you've done it through the level 21. Just try again and use different tactics. Elimination is good to see which one can be used to finish the levels. On a side note, those levels were created to be finished which means, there is a way to do it.
