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.

It seems that so far, Monster Hunter Wilds is matching Baldur's Gate 3's pace two weeks after launch, or is at least close to it.