Complex sentence: The player moves the character about the bottom area of the screen with a trackball and fires laser shots at a centipede advancing from the top of the screen down through a field of mushrooms.
Split into simpler sentences: The player moves the character about the bottom area of the screen with a trackball and fires projectiles (something like laser shots) at an advancing centipede. The centipede advances from the top of the screen down through a field of mushrooms.