I finally found a Legendary fish near the ruined house during Winter. Is there any technique for actually catching the damn thing? It almost seems impossible.
Fish O' the Sea is a good dish giving +3 buffs. I prefer the trap bobber which decreases how quick the fish can escape but the cork bobber is good since it increases the size of the green bar. Use bait so you lose less time waiting for fish to bite. It's much easier when you reach level 10, but good luck!
If you are still having trouble after using bait ect. you can always download a easier fishing mod ^^
Luck plays a large part. Sometimes legendary fish swoop up and down so fast they're literally impossible to catch, but sometimes they're a lot more reasonable.