Just playing the game and following the story line and side quests should earn you enough coin to get this achievements. There are however two things that can boost this.
- In the wind valley (present), hand over the flying to the inventor. Travel to wind valley future and it is a small village with a shop where you can buy multiple 'lucky necklace' which doubles your coins for each necklace for all coins gained through killing monsters and farming bushes.
- Collect the three epic ingredients and deliver them to Wok Donalds in the future for 6666 coins.
1. Mammoth steak, located in the grizzly pass in the present (behind the waterfall)
2. Golden Apple, located in Bluff in the present
3. Cheeseless pizza, can be found during the demonia campaign in the past