To increase odds of breeding a rarer dragon, how should I level my dragons? Should level 10 dragons be raised to level 15 in any particular order or grouping (e.g. plant dragons first, cold second, etc.) or better to raise one of each to level 15 first? I'm also trying to take into account the max amount of coins earned in each habitat.
Level 15+ may increase your chances of breeding the rarer dragons (using the enhanced breeding cave or the Island), Level 20 dragons do offer an enhanced percentage success. To raise dragons above level 15 uses an exorbitant amount of food. Unless you have virtually unlimited cash / food the best way is to build up your food reserves (which also adds XP points) but don't use the food. When a rare dragon is released, check to see the combo required and use the food stored to raise the level of the two required dragons as much as possible. Some dragons are used to breed more than one rare dragon which does help. I have a couple of dragons at Level 17 and each 'stage' is taking over 300K foods ie. In excess of 1m foods to raise one dragon from Level 17 to 18. Needless to say to raise it from 18 to 19 will require even higher food per 'stage'. Raising levels according to available rare dragons must be better than randomly increasing a dragons level imho.