The first time you encounter a Pinataface will be in Los Manglares on the way to the Jade Temple in the last room before the temple's entrance. This will trigger a Lucha! fight and it will be the last wave. You'll know them when you see them, they're huge skeletons wielding a mace and, well, wearing a pinata on their face. The attacks can be dodged and you must hit them in the face. Once it's defeated, the achievement will pop.