There is an area in the castle where there is a large glass case with ammo and weapons in it. A rocket launcher will be included. Take it, and save it for the Salazar boss battle.

Right before you enter the room where you have to fight Salazar, there will be a merchant. If you do not have a rifle yet, buy one. Then go in to face Salazar.

Salazar mutates into a giant monster that has a large yellow eye. Stand back so you don't get hit, and shoot the eye once with the rifle. It will severely hurt the monster, and a part of it will open up to reveal the head and upper body of Salazar. Go to your inventory, select the rocket launcher, and shoot Salazar. It will kill him immediately.