This is the final boss of the game and is accessible by climbing to the top of the snake temple after doing all the other story tasks. Some prep I'd recommend is having all the blue, green and red trainers skills maxed out. Blue's meditate is invaluable, red's 50 and 100 hit bonuses make life easier and green means you do more damage with your specials.
This fight has 2 distinct parts. The snake heads and then Salvador himself.
1st stage with the snake heads you are fighting two initially and they generally alternate in firing beams across the screen. The beams themselves aren't too difficult to avoid and when you get hurt a lot you can quickly meditate back to health in the downtime between. My biggest tip here would be to focus on getting both snakes down to low health and then finishing them off together as closely as you can. One snake on its own gets in a frenzy and seemed the far bigger threat.
2nd stage is Salvador himself. Focus on special colour matching to his shield but not essential to give yourself an advantage. It can be worth getting in an uppercut to launch him first and then switching to the colour move you need to start your combo. The stage gives you good opportunities to roll away to the other side and meditate. His dive and charge moves have clear indicators and wind up times. His charge move is undodgable so be sure to jump. Given some careful meditations and a few juggles he'll be down before you know it.
I had 3 attempts at the first section and then was able to do Salvador himself first time as a guide. Hopefully with the knowledge about snakes you'll be even quicker!
I also did this at the same time:
But bear in mind Jaguar takes more damage against Salvador and is a paid dlc character.
After the fight is finished you can continue the campaign where you left off and do the final fight again if you haven't 100%ed the game yet.