Hoard

Your dragon must be leveled up to Rank 15 to get this trophy. Complete levels and earn badges to level up. Look under Badges in the Rewards menu to see each badge. Here you can also find your current rank and how many more points you need until you level up. Each badge is worth about 100 points, and you get 50 or 100 points for each medal you earn on a level. When you play a level and earn enough points for the medal, you will get the points. You get 100 points for the first time you get enough points for the medal. You will also get each level of the badge. For example, if you get enough points for the Gold, you will get the points for the Bronze and Silver as well. You will also get 50 points if you play a level you already have the medal for and get enough points for the Bronze.

Points needed for each rank
Youngling Rank 2: 500*
Basic Dragon Rank 5: 3,250*
Advanced Dragon Rank 7: 9,500*
Veteran Dragon Rank 9: 12,500*
Wyrm Rank 11: 19,750*
Great Wyrm Rank 13: 27,500
Legendary Wyrm Rank 15: 37,500

* - Not exactly sure about these points. I started to keep track long after starting the game.