Royal Garden G20:
Make 3 consecutive matches with flower tiles.
While playing in ANY game mode, complete the following steps:
Step 1) Match any one pair of flower tiles
Step 2) Hit "Undo"
Step 3) Match the same pair of flower tiles you matched in step 1
Step 4) Hit "Undo" (again)
Step 5) Match the same pair of flower tiles you matched in steps 1 and 3
This will unlock the Royal Garden achievement!
As a bonus, this method will also unlock the Hat Trick achievement (match three identical pairs of tiles in a row) if you haven't unlocked it already. Special thanks to Basileus for pointing this out!
And that's a wrap. Enjoy your achievement(s)!
NOTE:If you are using the default tile set, this method did not work for me when matching the tile with a yellow flower/blossom on it. I would suggest attempting this achievement with any of the other flower tiles!