After you complete the Templo de Obsidiana and emerge from Infierno through Bonus Street, you will have the skills you need to reach this mini-boss. Head east in the Badlands to an area just west of the Villachula entrance in which you can drop down (or jump up depending on your approach) between spike walls. Halfway down/up this hole will be a blue block on the right you can dash punch through. This will bring you to the entrance for Pico de Gallo (Grindiest Timeline).
After some brief dialogue with the villagers, you will start to make your way up the mountain to the east and will see an enemy. When you approach it, however, it will simulate a turn based combat. There's only three options to choose from: KO Headbutt (single target), Dash-Punch (all targets) and Heal. I played through each of the 4 rounds by Dash-Punching until all weaker enemies were dead then Headbutting the strong ones that were still left. I never came close to dying and never had to heal once. The final round is the mini-boss El Trio de la Montana and once he is defeated, the achievement will pop.