How to Train Your Dragon

Challenges are the best way to accumulate the gold you need for this achievement.

A couple things to keep in mind:
Try to purchase as little as possible until you have your 10,000 gold and unlock the "Viking Gold" achievement.

After you collect the 10,000 gold, go and save your game.

Now go to the store and purchase all the recipes to get the "Cooking Master" achievement.

If you want to keep the recipes, go and save your game again. If you don't care about the recipes, purchase every ingredient you can from the store - especially the rare ones.

Return to your house and enter, but DON'T SAVE - just exit back out to the village. This will restock the store.

Go and purchase everything you can again. Repeat this procedure until you spend 10,000 gold and unlock the "Shopping Master" achievement.

Now reload your game and you will have all of your gold back.

The reason you want to do this is because after the end of the story mode, you need to go to the viking in the Wild Zone and fight his Legendary dragons. This costs money. A LOT of money. Doing this now will save you hours of playing challenges to accumulate enough gold to level up your dragons the final three levels from 22 to 25.