After heading through the door at the lower level of the first room, head down the stairs to reach the B.O.W Lab. Grab any ammo you may need off of the floor. The characters will talk a little bit about Birkin, the doctor who they have been sent to collect along with his specimens. Soon, the conversation will be interrupted by invading enemy forces. An explosion will rock the facility, and enemies will run into the area. Take cover on the dividing wall and shoot the enemies below you. Pass through the door in the back of the area when they are all dead.
Use close quarters melee attacks or a Shotgun to eliminate any stragglers who may still be alive on the lower level. When melee attacking, you can either hold down the melee button for an Execution, or hit the action button when you stun them to perform a Brutal Kill. Search the far left side of the area to find a Frag Grenade. Head through the door towards the center of the room. More enemies will probably pour in through here. Take cover again and pick them off with a Pistol or Rifle of some sort. When all of the enemies have been finished off, you will receive a transmission from HUNK explaining your next objective.
After this message, the door to the back area will be breached and a group of enemy soldiers will burst in from behind you. Take cover again and pepper them with gunfire. Alternately, if you are fast enough, you can take this entire group out in one decisive blow by tossing a Frag Grenade into the doorway as soon as it opens. Now that all of the enemies in this area are dead, take the time to search for supplies and hidden items. You can pick up any ammo that you may not have needed earlier by searching the floor towards the beginning of the area, and you can grab Intel from the computer table under the stairs to the upper left side of the arena. If you’d like, you can also switch out your primary weapon here for one that the soldiers have dropped. You may be tempted to do this for a Flamethrower that sits in a weapons case towards the top of the area. Head to the upper level and follow the catwalks past where you entered the area to find the crate. Shoot the lock off with your pistol to get this special weapon. Just note before you do that you will be fighting human enemies for the rest of the level, and this weapon is best used against the infected.
When you have taken all you want from the room, climb the stairs near the exit to find a Security Camera perched on the wall above the exit. Shoot it, and head down the corridor on the bottom floor.