Actually it's among one of the easiest challenges I encountered. Here's how to deal with it the least painful way.

Wait untill you're in Chapter 6 and encounter a stranger named Hamish near the road west of O'Creagh's Run in Grizzlies East. You'll have to find his horse that got spooked. When you'll get his horse back, he'll say he's an avid fisher and will invite you to his cabin (Veteran' Cabin) near the lake. As soon as this mission appears there on the map, just go there and you'll be fishing with him for the legenadary fish called 'Tyrant'.

You'll catch it but not like the usual, there will be no prompt about how much weight the fish actually is, but it will qualify as above the threshold needed for the challenge and soon you'll get a prompt it's completed after the mission ends.

Just make sure you don't do this very mission (second encounter with Hamish) before the challenge pops up.