This achievement unlocks naturally as it's likely that you will match 20 unique pairs in a row. However, if you want to focus on what you are matching, I would suggest that you use any of the other themes aside from the Tranquility theme. The other themes have unique symbols/drawings representing each tile found in the Tranquility theme (for example, in the Cosmic theme, the red dragon is represented by the circle, green dragon is represented by the triangle and the white dragon is represented by the trapezoid). Also, go for one of the puzzles in the Hard and Expert categories. These will increase your chances of unlocking this achievement.