The best, quickest, and easiest way to get this achievement is by simply buying 25 fire dragons which cost 100 gold and 30 seconds to hatch. You can just sell them once they hatch giving you back 50 gold.
*MoneyShot360 also recommends this*
Before doing this, make sure you have a room open for a fire dragon. Place the dragon after hatching, pet 3 times for 20 gold each and then sell. You end up gaining 10 gold per dragon instead of losing 50 gold.
3*20 gold for petting = 60 gold
50 gold for selling
100 gold cost to purchase
While your at it, might as well do this 50 times to get the gems from the in game achievement.