To get this achievement, you need to give Gunther the bird hiccups.
To do this, you need to feed him five fish BEFORE you lure him down from the cliff face.
Once you've equipped the fishing net to the boat, go to and back from the lighthouse, picking up the fish in your net. Then go back up to floor 0, out onto the cliff, and feed gunther the fish.
Do this five times.NOTE: Don't forget to talk to him multiple times between and after each fish-giving, as this gives unique conversation lines that count towards this achievement: