Click on the shapes to buy them to fight. They go to the arrow that you select. Click on an arrow to select it. Squares are the cheapest, circles have the most health, and triangles are the quickest.
Everything by me except for part of the list script, which is by @Cyclops1. I shared this even though it is not quite done so that maybe someone will be able to figure out the glitch, which is that after a little bit, when you click on the shapes to buy one, it gives you two. I will follow whoever figures out how to fix the glitch.