The title of this is actually a bit misleading, a more accurate description would be
Complete the 25th level meaning that this (& any other level specific achievements) don't count the so-called "Secret Levels" which are the ones that start with a letter like e.g. V1.
Now as you can't get to the next level before you've completed the one before you can't have an earlier level incomplete so:
Tl;Dr:
Complete the 25th LevelGood Luck
Edit: You do
NOT need 3 Stars to complete a level