Level 17
Looks strange, but you walk on the sticks. Run right first, then start shooting the green balls. The small versions will often get stuck between two sticks, but quickly make it back out. Don't step on the area between them if there is one in there. On the whole, not too hard.
Level 18
Shoot straight up from the start, then run into a corner with the power-up if it was good. The medium-sized balls won't bounce into here, so it's fairly safe. Take them out slowly, then open up the top and get those as well. They are slightly faster and harder to predict with the many areas it can bounce off.
Levle 19
Let the balls come closer first, then dodge to the left side. Taking out purple is easier, but the pink one is quite easy as well. Both are slow but watch out for the small pink balls when you are up on the steps.
Level 20
Start with an easiest one, like the second from the left or right. You have enough space to run for cover. Try to open up the second glass panel as well, as the balls will bounce higher and are easier targets. There are lots of power-ups, so use them wisely. If you have a sandclock, open up another ball because it will be split into parts by you within seconds.