I had hard time to find the last one, but thanks to PowerPyx Youtube chanel i found it.
See the video below :
1 - After the first whirlwind you meet, stop and do a 180 turn you will see it just in front of you.
2 - In the green liquid room, as soon as you enter look at the right corner of the room. It's too dark to see the gear but light the corner up with some flames you will see it !
3 - Once you are on the bridge before the Supercharge, lift the bridge up by flaming the golden thing. Then go to supercharge, jump at the top of the bridge and you will find a small room. From this room jump to the lower plateform with yellow gems and then look at the door in front of you (when you land, ont the plateform..) you will see the gear on the right. Jump to it and destroy it.
4 - Ok go back to normal path to the end of the level until the pink room. As soon as you enter the room, stop and do a u-turn you will see the gear on the wall.
5 - Exit the pink room, when you step down, stop, u-turn, on the step.
6 - Go to the end of the level. Facing the Home Portal, do left, left again, the gear is behind the pillar.
Hope it helped, the video is not mine i don't take credit just want to share here what i used to unlock the achievement.
Edit : As Ketley1996 pointed out, you must find all six gears in one life, if you die you have to destroy again the gears you already destroyed.