If you only have one Pure Dragon, instead keep breeding a Pure Flame and a Pure Terra Dragon together, until you get either a Legendary dragon or a second Pure Dragon to breed.

You'll have to wait longer on average each time, but when you fail to get a Legendary you might end up with a cool new dragon. If you'd rather get new, exciting dragons, just keep breeding combinations of dragons that both include Pure elements, for instance, Kratos Dragon + Pure Dark Dragon.

The downside of this strategy is that you'll end up with another Pure Dragon 94% of the time. For me, a Legendary (Rarity) dragon, Should have either the ability to crit 4 or more elements, or have the ability to crit (At least) 1 Advanced element, so it can be better than Empowered Epic dragons, but I find nothing of that in the Pigheaded dragon, it can only crit 1 Element from Basic elements, and 2 Intermediate elements, and it.

Other combinations will take a longer time on average, and still only have a 6% chance. If you're trying to maximize your chances of getting a Legendary dragon, breeding two plain Pure Dragons together is your best bet, since each attempt will take 12 hours of waiting. Any time two dragons with a Pure element are bred together, there's a 6% chance that one of the Legendary dragons shows up.

Breed two Pure Dragons for a chance at a legendary.

Dragon city legendary dragon