This is how I unlocked this Trophy - hopefully you will be as fortunate!
On the 1st World - just before you get to the Iron Outpost (the games first Dungeon), you can explore an area with a glowing key-type door. At this stage you cannot access it, and the game tells you to leave the area.
Move through the area and you'll see the first 'fishing area' - here I caught my Epic Fish.
I had not purchased any additional Rods, Lures, Bait etc. - and after my 3rd cast I got a Jub-Jub Fish.
As the Trophy name suggests - Beginners Luck!?