Lol if he/she had done that they wouldn't be out of letters imo. I've completed every level 3 times today and i still have 999+ letters and i haven't been playing for 2 days lol
With the right deck, it isn't that hard to do. This event gives you 3k DP for LP on the brink, so I just use last gamble. That's an easy 5-6k every time you pull it off. If you can average just 5k per duel, it'll only take you around 600 letters to get 9999 coins, and you'll often get a lot more than that.