It doesn't.
Got the Sky Dragon today breeding lvl 15 Nirobi x lvl 15 Terra Dragon ;)
Legendary and Electric! Any level works, as long as its legendary!
wow. I used Legendary and Volcano, hoping its anyone but a bat and guess what. its a bat. ARGGGGGGGGGGGHH!:(
The only way to breed the Sky Dragon (at the moment) is to breed two Legendary dragons together. I tried to breed this dragons with all known combinations for multiple times (70+ attempts): all failed. The second attempt Legendary + Legendary was successful.
Got it by breeding Justice or Sun (I'm not sure) + Fluorescent, both at lvl 10
Just use star dragon + terra....Only four poosible outcomes.....chameleon, star, terra and Sky. It worked on my first attempt.
I got it by Gummy and Armadillo(Lvl.10 1st try)I actually wanted supersonic.