Part 2 to my Gun post because I ragequit without beating the boss but I returned later to finish it. The boss can actually hurt himself by throwing dynamite into the geysers. It's kind of a blessing, but it's baffling how incompetent you need to be to make the final boss hurt himself by accident. After you beat him, he actually has a second phase where you need to shoot him in the head. Since he was indestructible in the first phase and he's still wearing his armor, you'd assume he still doesn't give a fuck about headshots, but now they do and they stun him. This triggers him to throw dynamites and you need to use deadeye to slow the game down and shoot it while it's in the air, and it explodes and collapse the cavern on him.
If you shoot the dynamite in midair but not quick enough, they won't damage the pillars/cave so it does nothing. After about a minute, the game give you hints like "you need to anger him" "you need to make him throw dynamite" "you need to shoot his dynamite". It doesn't mention headshots, but that's a decent hint. If you run out of deadeye, you're basically fucked, but the devs seem to have noticed this last minute, and will send 2 guys as reinforcements to kill and refill your gauge with. Sometime they don't spawn so you need to wait. Sometime your deadeye is full and they still spawn. 2 more can spawn right after you killed the first ones, and then 2 more, and 2 more. The reinforcements also appear behind your back when you are trying to get cover from the boss. You can't melee them or else they melee you too and their melee kills you in 2-3 hits. If you're quick enough, you can also cheese the boss with a headshot, then his dynamite, rinse and repeat, so that's also good. If you die during the second phase, you restart the whole fight.