The five platinum generals are the most powerful aliens apart from the Alien King in God of Death form. In fact, the five-star platinum generals can control Guthix's Gaze, which caused them to defeat you at your first encounter.
-The first and second platinum generals aren't nearly as strong as the ones above them. The second one is designed to manage the aliens' technological needs. The first is made to be overly-confident of his ability to intimidate their opponent and make the others seems unbelievably powerful.