Super Mega Baseball 3

Fixed as of the 06/05/20 patch, no longer unobtainable:
You need to finish a standard regular season(can work in franchise) with your players being at the top of the 'League Leaders' list in batting average, home runs, and runs batted in. The results will differ for everyone but aim for at least .500 batting average, 10 HRs, and 20 RBIs. I recommend a power hitting team(since HRs count as RBIs as well) but if you play at a low enough ego it shouldn't matter too much which team you choose.

You'll probably get this on the way to other achievements(undefeated season) but if you're looking for just this one you can play 2-3 games at a very low ego, forfeit the remainder except for the last game, check the league leaders to see if any of the CPU players are close and either forfeit the 16th game or play as needed. There is a minimum of at bats needed for the batting average to count and it's determined how far along in the season you are. After your first game almost everyone should register on League Leaders but by the end of the season you'll need a minimum of at least 45 ABs or so. This is why some people with a high batting average eg. 1 hit from 1 AB therefore 1.000 batting average, will not show up on the leaderboard, so if you don't have the necessary HRs/RBIs on game 16 or the ABs to have your batting average register you'll have to play the last game out.

The achievement popped for me when I hit start playoffs, not necessarily at the end of the last game.