Try this...everytime you got yourself an ultra rare, super rare, or even rare dragon breed fire and fire for several times.
As for me, if I get an ultra rare, i usually breed diamond with fire for about 15 up to 20 times, then I sell the fire's egg and trying the one you want. The reason why i do this everytime is because I never got rare or super rare dragon twice in a row, so it is kind of refreshing your luck. I did some experiment to get 2 crusader ( fast fowarding the 1st one & go for another to see how many fire failed result before i will get a crusader again )...oh you need 2 nest for this...and the result was 34 failed fire before I got myself a crusader again. Hope it helps