Go in the helicopter. When the pink tentacle appears from its back, pump it full of lead. Then, hit the mouth part. When you get knocked to the ground or near it, blow up one of the barrels and watch it burn. Repeat this until an intermission sequence starts.
After killing all the snipers sitting on top of the building, go back inside. Get warmed up and run to the left of the building where the snipers where sitting. When you go down the trench, a few intermission sequences will play. You will now be in a helicopter. You do not need to have any life to defeat this Boss. When you fly towards the creature, you will see a pink tentacle appear. Shoot it. After shooting the tentacle and it sucks back into the creature, aim the machine gun to the ground and shoot the four groups of barrels. You can shoot all the barrels on the first pass, but make sure to shoot the tentacle first before going for the barrels. After all four groups of barrels are on fire and burning the creature, you will make about three or four passes shooting the creature and the tentacle.