I didn't even know about Shedinja until recently and I never used it to catch legendaries. I recomend paralyzing, lulling them to sleep, or confusing them to make them stay cooped up in the ultraballs. As for catching Latias or Latios, I never used shedinja there either! I used Crobat and his Mean Look, and Wobbafett and its Shadow Tag. I eventually caught Latios this way, and I still never found Latias, however, if you want to do it the way you want, it isn't in my power to stop you, and my post is only oppinion anyway.