To feed a Dragon to level 15 you need a Temple for each element the dragon has so you can level it past 10.
e.g. Fire dragon requires the fire temple while a smoke dragon requires the fire and wind temple
To reach level 15 requires 37,028 food.
The fastest way to rush this would to be use the fire dragon. If you are going for other achievements within the game you will end up with lots of dragons at this level. You can accumulate food quickly by completing campaign missions, arena win streaks, event rewards, as well as farming the food normally.