To catch a whopper for the Catch of a Lifetime quest, you must equip and use one of the new lures Kanya gives you after completing Fishing for Flavor: Tuff Joint Bait or Duster Rig. For my catch, I used the Duster Rig.

To get a Whopper in Monster Hunter Wilds you should look for pools of fish actively thrashing about with large fish swimming between them during the season of Plenty. Now, you can catch a Whopper another time if you want to but this season is the easiest way to ensure you can find one.