I actually find Salazar to be quite easy once you get your position right. It takes work and many weapons changes, but it can be a breeze when you get into the flow. Running to the sides and shooting the tentacles with something strong (like the BB), then the eye with something fast (TMP, handgun) and when the shield opens, hit him with your strongest weapon (I normally use the Bolt Action Rifle, since it already got the exclusive by that point). Just watch out because the tentacle will come back after a couple of minutes.
I if I gotta say that the hardest boss for me is Krauser, since his attacks are pretty strong, and even if you use the knife, it's still a little risky, so I go with that. (Also Saddler, if you don't use explosives and shoot him in the little eyes, can be a little of a pain the ass. But explosives breaks the fight)