Salvador is the final boss of the main game. If it is your first playthrough, you will encounter him through the door at the top of Templo de la Serpiente. If you've already beaten the game, reloading that file will put you very near this door making it quick to get to the boss fight. There is a save point shortly to the left and down of the door where you can change costumes to Jaguar. It should go without saying, you need this DLC to have this costume.
For the fight itself, he is not too difficult. You will first battle two giant serpents, each with their own hit bar. You can use any attack that you prefer while dodging their various moves (borrowing underground then flying up, launching fireballs, falling fireballs, ground fire, shooting purple/white rays, etc.).
After they are defeated, Salvador will jump in to fight. He will have a shield and three serpents on his back to start things off. To disable his shield, use the corresponding chicken power (power slide for orange, pollo shot for purple). He will then fight you using more traditional shields (blue, red, green, etc.) which can be broken in human form with the corresponding attacks. Fight him in your preferred style when the shields are down while dodging his attacks and he will quickly be defeated. You can then pause and quit once the achievement pops to skip the long end game cut scene.