When you'll have to start off by reducing the size of a circle in order to adjust the number in the center of the screen keep in mind that once reset, you can start building the number back up to 100.
Black circles with a gray cross in it represent immoveable objects, and you'll need to work your growth around them.
If one half of the circle you press is highlighted, that means it's tethered to another circle. Find out witch one and do them both at the same size.
When you have spinning saws slightly increase the size of a lot of circles, so if there's a collision you won't lose a lot of hard work.
If a wandering ice bubble has frozen a circle solid, simply hold down the frozen circle until it's become large enough to burst through the frosty layer.