Well, let's just say this is one of those "Be sure to check the body" kind of situations. Firewings went down over the SOX, yes, shot down by fighter planes, but no one ever did check to find her body. Haeslich(sp?) got gunned down by Twist and his crew back in the 50s, but again, the body disappeared in the river. However, it is possible for metahumans to take down, even kill, adult dragons. It isn't easy, and you're going to need a lot of heavy guns (and luck), but it is possible. Taking down a Great Dragon, however, is another story altogether. Difference between an adult dragon and a great dragon is like the difference between High School baseball (maybe college) and the Major Leagues. For reference, the rest of us are middle school teams. For metahumans to take down a dragon, forget your A game, you need to be on that AAA game at least, and the dragon needs to be having a Bad Day. Metahumans taking down a great dragon (without another great dragon doing most of the work) takes, well, Aztlan almost got enough together to pull it off, but even they couldn't finish it.
Unless of course you're Harlequin, and can stand toe to toe with Ghostwalker and fight him in fragging silver plate mail like something out of a bygone world.