1. Open the red gate and collect bonus coins hidden in the left side wall (5 coins)
2. Let Loof push the bottom and the center boulder to the right and follow them collecting the bonus coins in the top and the right wall.
3. Enter the red teleporter from above and return through it immediatly
4. Collect bonus coins (there are two more hidden in the bottom wall) and push the right boulder into the yellow teleporter.
5. Let Qookie open the purple gate and push the remaining boulder up (!)
6. Pass the gates, open the green gate and push boulder in the yellow teleporter
7. Collect all keys and take Qookie and Loof to open white and light blue gates
8. Exit!
P.S.: There are three more bonus coins behind the Goldstar.
Myself I did it quite differently. (I do not explain bonuses, though)
1. Open the red gate. DO NOT COLLECT THREE KEYS for now!
2. Let Qookie open the blue gate and Loof push the bottom boulder to the right. (Move Stinky out of the second column, too)
3. Let Qookie open green (or was it pink?), middle gate and push the center boulder right.
*It will stop in Stinky's room and hit a conveyor button.*
4. Have Stinky to collect three keys and enter the yellow teleporter from the right and return back to his room.
5. Have Loof to push the bottom boulder right.
*It will go into the yellow teleporter and in blue.
6. Have Stinky to push the boulder on the conveyour button down, and stop yourself on a button. This way it enters the red teleporter and seems to be stuck...
7. Let Loof to enter the yellow teleporter from the left.
*The second boulder is now in the water*
8. Get two remaining keys in the teleporter-room and have Loof AND Qookie to open light-blue and white gates.
9. Exit.
Rest in peace, Kym. I hardly knew ya. Rest in peace, Marinus. A bright star, you were ahead of me on my own tracks of thought. I miss you.