I have an alternate way of getting the achievement, though it requires you to be further on the game than the bandit mantle method.
Unlike the research point one, this achievement requires you to have 1 million zenny at hand at one time.
As you progress through the game, you'll begin to unlock investigations. These are generally randomly generated, and even have modifiers to the quest.
One of these modifiers is money earned. These are perfectly capable of breaking 50k+ zenny for a single quest in high rank.
Combined with the daily login voucher, your money gained from a 10-15 minute quest can easily break 100k.
Obviously if you want to use this method you'll need to do the mission solo. Taking multiple hunters into the quest will divide the rewards up between you all. I personally find that solo quests are easier due to the hp scaling of the monsters, however.