When you have a big or highly-energetic fish on your line, tap REEL at a slow rate. Thisgives the line just enough time to keep the line easy, and enough time to hold on to the fish without it snapping your line. This should work for any fish of any size, no matter what type of challenge. If the line gets very tense, release it and let it go into the yellow zone for about a second, keep tapping it at a slow rate.
Use the Sonic bait or Deep Crank and cast it to the left of the palace's front door. Reel it all the way in until it sinks to the point where it is directly behind that armored body's head. Keep reeling it in, and if your bait hits the back of the armored head, it will fall off.