There's 7 levels that are locked while going though the game.
1-24, 1-25, 2-23, 2-24, 2-25, 3-24, 3-25
After finishing chapter 4 you unlock these levels
Now you need to complete 3-24 and 3-25 to complete Chapter 3 for this achievement
I made a walkthrough for these levels in my video @ 7:45