Blake enters the Weapons Laboratory in order to retrieve a keycard in order to use the elevator to reach the surface. However shortly after he entered the lab, a large Rupture emerges from the ceiling and, together with a nearby Walker, attacked Blake, but he was able to kill it by shooting it several times and finally burning it.
When you enter the room a little cutscene will introduce you to the third thing boss: this is the hardest boss in the game by far if you don't know the proper technique to kill it. Inside this same room is a Walker held in a glass container beside the boss. Firstly, when you enter the room, run to the back of the room and circle around the Rupture, then crouch and position yourself beneath it; it won't be able to hit you.
From here you can shoot the boss until its health becomes red, and then you should blow the tank with the Walker and try to kill it. Remember, don't leave that place where you are crouching or else the boss will be able to attack you. Try to kill the Walker from the same place where you are, keeping it at bay with your flamethrower. If the Walker comes close to you use the flamethrower to fend it off, of if it stays away then the only thing to do is to throw a flame grenade.
When the Walker is dead, its time to kill the main boss. Throw a flame grenade right underneath it and run quickly back, keep on retreating so that the grenade doesn't harm you. If the boss doesn't die with the first grenade, then the second should definitely kill it. When it is dead, a new objective will be added to your list which is simply "escape."