This achievement will require you to go fishing out front of Morning Glory but first go to the public market in Okinawa and buy 10+ shrimp bait just in case you don’t get lucky. Then go back to the fishing rod on the beach in Morning Glory. Keep entering/exiting the mini-game until fish spawn in the 140-150m range (Tuna will only spawn at that range). Cast your lure with shrimp bait ensuring it lands on the fish in that area. Don't touch the line and wait until A appears (it may take a few tries). It appears for less than half a second so be focused on that prompt.
Once you've hooked the fish, it'll take 2-3 minutes to reel it in. Ensure the line doesn't go above 75% tension and point your rod in the direction the fish is moving (left or right). Just be patient and don't rush it while keeping an eye on the tension meter.
Also, note the tuna you catch can be used for the sea bream sub-story so I'd recommend hanging onto it until then if you already haven't just paid the 300k for it.
Here’s a video of when I got this achievement: