This will likely be one of the last offline achievements you will earn. To earn this achievement, you will need to unlock all 12 fantasy characters. You can unlock them by defeating them in boss battles in Blitz Gauntlet.
Each Blitz Gauntlet consists of 12 games, 3 of which are boss battles. Therefore, to unlock all 12 characters, you will need to complete the Blitz Gauntlet 4 times for a total of 48 games. Thankfully, each Gauntlet can be played on easy difficulty, and you do not have to worry about facing the same boss more than once.
Each game takes about 20 minutes, so it will take about 16 hours to complete this achievement. It would be smart to go for the rest of the offline achievements while going through the Gauntlets.