Here ya go. Just remember it's theory but it has worked for some and the breeding times usually aren't too long. I loved that about it. Anyways this is what I did and just copied and pasted from wikipedia.
Step by Step Easy Guide
Edit
1. Breed a level 10 Sonic and Lava (in that order) until you get a
Firefly Dragon.
2. Name that Firefly "FF10x4" and feed it to level 10.
3. Breed a level 10 Ice and Flower (or Plant if Flower isn't giving it to you) until you get a
Seaweed Dragon.
4. Name that Seaweed "SW10x4" and feed it to level 10.
5. Breed "SW10x4" and "FF10x4" until you get a new Seaweed Dragon OR Firefly Dragon (or a rainbow as your % of obtaining one has improved at this point).
6. When you get either of the dragons above, name them FF10x7 or SW10x7 (depending on which one you got.)
7. Feed the new dragon up to level 10.
8. Replace whichever new dragon you got from step 5 with it's parent dragon that you were breeding with from step 5. i.e.-(if you got a
Firefly Dragon, replace "FF10x4" with your shiny new "FF10x7", if you got a
Seaweed Dragon replace "SW10x4" with your new "SW10x7") Now you've essentially replaced one "grandparent" with a "parent" and we need to replace the other "grandparent" now.
9. So... Breed your new 10x7 dragon with whichever 10x4 is left in step 5. i.e. - (SW10x7 with FF10x4 OR SW10x4 with FF10x7) until you get a replacement for the 10x4 dragon that's left or a
Rainbow Dragon (as your chances have increased again)
10. If/When you get a replacement for the older "grandparent" 10x4 dragon that's left. Then feed it up to level 10 and replace it with either FF10x4 or SW10x4, depending on the dragon you got. (If you want to take this a step further and get 10x14 dragons, go ahead, but I doubt the chances increase after both dragons contain all elements).
11. Now you should have two dragons that are 10x7 (meaning their parent and grandparents were level 10 AND they both contain all 7 major elements (Plant, Earth, Fire, Water, Cold, Lightning and Air). At this point you have (in my theory) the HIGHEST possible chance (not counting your level, which I believe increases the chance as well) to breed a
Rainbow Dragon.
12. Breed your SW10x7 and your FF10x7 (always in that order) and see how many tries it takes to get a Rainbow, it took me 3 (which makes me think this theory is correct) but it could take you longer. Post your results here. Thanks!