Two tips for Shapeshifter up to level 25
First, if you’re familiar with any somewhat sophisticated graphics software that have layering (e.g. Photoshop), it’s very convenient to put the shapes each on a different layer (in black), and set the opacity of the layers to be Difference. You can then just move around each layer until you get the right final shape. It’s important that you use “Difference” as opacity, since this does the even/odd cancellation for you.
Second, if you have pieces that are a single pixel, I find it much easier to set it aside, work with the remaining pieces to get something approximately right, and then fill it in with the single-pixel pieces. When you are not given such pieces, you can also “create” your own by canceling two shapes, e.g.