Had a day where nothing was happening at work so I spent around 7 hours fishing till i got mine (one of those from azshara, cant remember the name).
For the turtle i was at around 8k casts when i gave up, a couple weeks later just casually fishing for some foods and i got it....
I and two of my guildies switched from Howling Fjord, where we were for about a week, to Bloodhoof village in Mulgore and we all got the 15lb catfish within a 2 day span. Give it a try. We all fished right behind the one big building.
I spent about 8 hours total in Howling Fjord fishing for the turtle and the herring. Got the turtle and thought "fuck it", went to DM faire to finish leveling my fishing there. About ten casts in I get the fish mace for this achievement. Might be dumb luck but from what I've gathered the chance to get it on DM island is a bit higher (or it might just be that it can be cought in open water).
I can recommend the stranglethron fishing contest. I do it almost every week and on my forth run on the contest I got one of the rare fish that gave me the achievement, I forgot its name....
Rumor has it that this fish has a higher drop chance because its only available for 2 hours a week. give it a try
I had been farming this off and on for years, then I saw a random comment from someone that said they went to the lake in Teldrassil and stood on the prow of the boat and fished from there and got it right away, and I was in Darnassus at the time, so I thought, what the heck, can't be any worse than the hours I've already spent fishing for this frickin' achievement, went out there, bam, first cast. I could not believe it worked. I'm certain it was pure random weirdness, but still.
Then that Sunday I had the day off(usually don't) and remembered to go to the fishing contest, and won first place. By, like, minutes. Considering the number of times over the years that I've lost by seconds(several times by fractions of a second), getting first by a couple of minutes was surreal. But I got my title, I got my fishing pole, and I was ecstatic.
Went right out and rolled a toon named Choklitbalz.
I always recommend the stranglethorn fishing contest on this one. I know of two friends who get theirs while doing the contest for all the other items you can get for second/first place. I myself got mine there aswell. I believe it should actually have a higher dropchance simply because its only up for 2 hours every week and even if youre not lucky to get yours there you can still go for the other fish afterwards.