The first thresher you can hide behind the box in the left corner after you come down the ramp at the beginning and use it as a shield. The second thresher after the 2 cannons, actually appears when you pick up the 1st solar charge and sits over the water in the shadows near the exit, just don't deposit it until it's destroyed. Really all the ships are easy since there's something around tall enough to hide behind while shooting them down.