World - 20,000 Lums Under the Sea
Painting - Elevator Ambush
In the room directly after the big elevator fight, there will be a Sharkman throwing junk at you over a big pipe. Just stand next to the pipe and wait for him to throw ducks - usually every 3-5 throws is a duck. Hit 5 of these and POP!
Does not need to be done in one go.
There is also another area just further up that may be easier if you are having trouble in the pipe area.