Shoot down the spiders and complete all sixteen levels. Collect your ammo from the supplies board on the left side of the screen.
The fly in the top left side of the screen gives you a color to place on the grid below. Left click a block to change its color to the one supplied. If 4 or more blocks of the same color are connected they will disappear.
Amongst the blocks are ammo blocks, which will become available if they reach the bottom of the grid.
Remove the colored blocks fast enough from underneath the ammo blocks to keep a sufficient amount of bullets.
There are blocks with special weapons such as the fire-thrower and the atomic bomb. To use them simply click on the button of the corresponding weapon beside the ammo stack.