The hardest part of this one is remembering where the stupid Sharkmen are if you've already passed them without getting this achievement.
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!