To get this achievement, you need to give Gunther the bird hiccups, and talk to him before and after each fish you give him.NOTE: DO NOT forget to talk to him multiple times between and after each fish-giving, as this gives unique conversation lines that count towards this achievement.
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, which will unlock the following achievement:
After talking to him after he's got hiccups, if you spoke to him before and after each fish-giving, you'll get this achievement.