This achievement can be unlocked buy purchasing a second monster slot.
The second slot is 100k coins.PER COMMENTS THIS HAS BEEN PATCHED
Additionally according to comments the costs have increased greatly.
THIS WILL LIKELY BE PATCHED
At this time(launch day) it is glitched and will unlock without you having to spend the 100,000 coins by simply clicking on the "Unlock".
This doesn't save any time if you are going for the completion as you need to buy the remaining 2 slots in order to get