Under the warehouse through a door is your first real boss. As soon as the cut scene has ended keep yourself to the left of the room keeping your back against the door. There is a tentacle that pops out of the wall on the right. Shot gun the hell out of the boss then use the flame-thrower on it until it collapses on the floor.
You will have to shoot the little spider things as your killing the boss because there is a hatching bubble just the other side of the boxes your standing next to on the left of the room.