World 2 - Level 7
Guide the Chomper to Stinky's left to the purple button by walking around its cell as well as using the red teleporter. Next, open the green gate, and carefully move the reflectors into the positions shown. This will allow the fireball to take out an entire column of boxes. You might wish to move one of the reflectors to disrupt the path and stay out of harm's way of the fireballs once the four boxes are destroyed.
Now, using the red and green teleporter, guide the Chomper onto the yellow button. Finally, guide the last chomper onto the blue timer button and collect the Rainbow Coins.
A word of caution - once all Rainbow Coins are collected, the gates will also release all of the chompers. Move quickly and carefully to avoid their bite (and maybe collect some Bonus Coins along the way).