Lightning/Storm is better, with no chance of Hail. Breed at night, and try for other dragons as well (like, for example, Ice/Scorch can get you Rainbow as well, or Lichen/Crystal can get you Rainbow or Emerald). You might get Moon by accident. If you have dragons over level 15, use them to breed. The Epic Breeding Island gives slightly increased chances of breeding epics. Good luck!
I remember when I tried to get the moon dragon. Trust me, wait until you are a higher level and you will end up thinking of epic dragons as pests. If you accedentally get the double rainbow- BOOM, you have to wait for 48 hours before you can breed again. Trust me, eventually you will get it by accident.