@Twig The manual is available at AtariAge.com, but basically you either reduced the shield to where you could shoot it with your cannon (obtained by eating some shield and appears on the left) or you shot the fire ball it shot at you with your cannon. Be careful not to shoot yourself with your cannon. https://atariage.com/manual_thumbs.php?SoftwareLabelID=603
@Twig It probably depends on whether the video explained game play as it went or not. It's not really a complicated game, but neither is it completely intuitive.