World 6 - Level 4
Position Stinky, the box, and Loof, at the "S", "B", and "L" respectively. Let Stinky walk onto the conveyor belt, and immediately switch to Loof and place to box on the conveyor belt button. Switch back to Stinky and collect as many Bonus Coins as you want - you must however collect the one in the yellow circle.
Now use Stinky to set all Switchers to point in the white arrow directions (walk on and off to do so), then place Stinky at the white "X". Have Loof push the box onto the conveyor, and step left off the button. Wait a moment, then step back on the button and bring Stinky back to the white "X". Repeat until the box is on the purple button. Have Stinky collect the Rainbow coins, then set all the switchers to the black arrow positions. Exit, switch to Loof, and exit as well.