I know that bosses can be a pain in the ass especially if you fight them like a barbarian (e.g. all out attacks etc.). But the truth is that every boss in every game has its weak point and a certain to kill it making it almost childish or like every other enemy except bigger.
Salazar boss in RE 4 is no exception. A classic RE boss with long tentacles huge eyes disfigured bodies and all that with accompanies with sounds like a stomach that has intolerance in some food. Well the truth is that if you go from the front and start shooting you will end up dead quite easily. So here is the steps to kill it really really REALLY easily!!!!
1. Buy a Rocket Launcher from the merchant outside the boss' chamber.
2. Once the cutscene ends head to the right side of the balcony. From here you can avoid every attack it has to unleash. The only thing you have to worry about is the right tentacle attack but with hitting the prompts buttons or shooting twice with the shotgun it leaves you alone for a while.
3. From the right side aim and shoot the strange yellow-green eye that waves in the middle of the chamber. (if you do that from the front a huge mouth will open devour you and kill you instantly but if you do it from the right it will not bother you). After a couple of shots at the eye a huge cocoon will open revealing the remnants of the real Salazar.
4. Now if you bought the Rocket Launcher earlier a single shot is enough to just kill it already!!! But if by any chance you missed it attack it heart or brain with what ever weapon you handle the best. (I ve noticed that the cocoon shuts quicker when you shoot it with bigger weapons like the magnum or the sniper riffle, but maybe it has to come to a certain damage amount to close and with bigger weapons u need fewer shots for that. Specifically it takes 3 shots with the magnum to close again).
5. When it closes you just repeat the same process until it dies. Pretty simple huh?!
Tips: If by any chance you run out of ammo or recovery items, you just drop down from the ledge, kill with a single shotgun shot a few spiders, collect some ammo and then back up again.