The most effective way of breeding a scorch dragon is blue fire (lv 10 left) and a storm (lv 10 right). It has about a 20% success rate.
The egg is red with a turquoise lightning bolt. It takes 3 hours to breed, and another 3 hours to hatch. It can be purchased for 100 gems through the market. It gains you 2500 experience points when it hatches.
Other (slightly less effective) breeding methods include:
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The firework dragon is a limited edition. It is only available during July of each year. It has a high success rate with several breeding combinations, so it is not too hard to get.
Some reported combinations:
The firework dragon takes 6 hours to breed, and another 6 hours to incubate. The egg is red, white, and blue with stars. It gives 12,500 XP upon hatching, and can be purchased for 704 gems in the market.
You have to breed a Earth and Metal Dragon. I have a Scoria Dragon and I bred it using a Earth and Metal Dragon. It may take a long time though.