Board Thread:Dragon City Gameplay Questions & Answers/@comment-24401737-20140620100347/@comment-24568135-20140620124810

Pure Hybrids are your best bet. Short of that an Electric based dragon will work for the first two. The secondary element will have to be terra or ice for Supersonic. With Millenium it looks like you will have to have a Pure Flame or the Legendary Dragon to critical it without being subjected to a critical hit yourself.