This achievement will pop as soon as you purchase a second monster, being the Adventurer class the default/initial one.
The current prices for each monster are as follows:
- Gunslinger: 300,000 coins;
- Beserker: 300,000 coins;
- Magician: 600,000 coins.
I suggest acquiring the Gunslinger, the fox faced ranged monster, since a majority of other achievements and challenges can be accomplished by this class.
I also recommend being smart about how to distribute the skill points that you get after purchasing and leveling your new monster up, otherwise you might have to reset them more than 1 time later on and it will cost more coins. Make sure to read the "Combo Master", "Sacred Crown", "Marathon" and "Gotta complete 'em all" solutions to get a solid idea of how important the Gunslinger can be and what skills matter in order to pave the way for those other achievements. You might also intend to pursue the "Yolo" achievement with this character.
Last but not least, it's been said that the "The Collector" (to have all 3 classes of monsters unlocked) achievement only pops after purchasing the Magician as well.
If this solution is helpful to you in any way, please leave a thumbs up to let me know. Thanks!