Quickly circle around the Boss while shooting it with the machine gun. When it is low on health, use the flame thrower to finish it off. If done correctly, you can kill it without being hit. After killing the Boss, the Thing in the glass in the room will break out, and the door will be opened with a few guards.
This Boss has a sweet spot. If you get directly under it, it will not attack. When you get under it, shoot like crazy until its health is in the red, then burn it.
Place Peltola in front of you in the room, just before you enter the room where the Boss resides. Inch him into the doorway. When the door fully opens, push him in and fire at the same time. He will enter with you and fight. Use him for his fire power and to keep the walker distracted while you attack the big Boss. Peltola will die during the battle.
When you enter the room where the shooting ranges are located, there is a control tower with two guards. Kill them and link up with the engineer being held in that room. Whatever you do, do not shoot him. Have him fix the junction box to open a door that will allow you to save your game in that area. Then, get the key to the Boss in the weapons research room. When you open that door, there are two guards. Kill them, then push the engineer in front of the next door. Keep walking and push the engineer until the door opens. You will then see the creature in the glass container. Shoot the container and the creature will run after you. Keep shooting the creature, then draw him outside of that room. Once you have him outside, it will be easier to kill him without losing heath in the process. After killing that creature, save the game. However, make sure that you are fully stocked with ammunition, and will have some to come back to after killing this Boss. Once you save the game, load the game. That creature in the case is no longer there. You no longer have to worry about hitting the glass and trying to defeat both of them at the same time. The Boss is easy to defeat. Run sideways and shoot at the same time. Use the shotgun to get the Boss down to the red. However, if you get down to where you only have five clips remaining, switch over to the machine gun. You will need your shotgun ammunition to complete the game. After killing the Boss, there will be a few guards that will come after you. Kill them, then the building will begin to blow up. Run back the same way that you came in. When you get to the ramp, you must kill two creatures and three more guards. When you exit to the ventilation shaft back to where you started the game, must kill four or five warthog-looking things. Make sure you have a good supply of fuel for the flamethrower. When you get onto the ledge leading you to the elevator, the creature will run up the hill towards you. Run back to the ventilation shaft and crawl in. They cannot enter. Shoot and burn them from that ventilation shaft. By doing this, they will not take any life from you.