Here is the full list of all 14 Millie achievements.
Reach the aviation school
Guide Millie through 4 differently coloured teleports at the same time
Use the teleport 20 times in one level
Finish 8 of flying levels with 3 stars
Make 10 turns on ice in one level
Forcefully enter the one way street 5 times in one level using the beacon light
Collect all possible shoes in the whole episode
Complete the whole episode with 3 stars in every level
Use 3 distinct hammers on 3 distinct chipped walls in one level
Complete 5 levels within maximum time
Use the scissors 10 times in one level
Use the clock 10 times in one level
Finish 5 levels without using the clock
Complete 3 levels in a row without restarting or going back to menu