The 2nd hidden level is found on level 13
Achievement Walkthrough:
1st Hidden Level (Level 4): 2:00
The Unknown Character (Level 7): 3:10
2nd Hidden Level (Level 13): 6:00The Duck (Level 14): 6:40
The Snow (Level 15): 7:15
The Arcade (Level 20): 10:10
3rd Hidden Level (Level 24): 12:30
The Sword (Level 27): 15:05
Hidden Horse (Level 29): 16:10