A good tip (From watching a youtuber play the boss fight) Is to take out one of the tenticles on the sides then just chill out by the side, You will not recieve a lot of damage and you can just chill and take easy shots at him.Hardest is probably Salazar. For some reason I had extreme difficulty beating him on my first playthrough
To me the hardest was I think Verdugo (Salazar's right hand) I thought it was difficult since he was fast (until i foudn out about the canisters) and with the infinite rocket launcher, he was dead quickly