Firstly, IGNORE ALL THE OTHER MONSTERS!!!!! Do not waste your ammo on them. Parker will shoot at them, and by the time the fight is over, maybe he'll have taken one down. Only Attack if they're blocking your path. Secondly, as spinny said, shoot the red air tanks! These and the charged MELEE attacks are the most powerful moves you can do to him. But even before the boss battle starts, you can explore the little arena area. Take this opportunity to grab all the ammo (of which there is plenty!) Finally, ignore what spinner said, the sniper rifle is extremely important. You don't want to be close to the canisters when they explode, and the sniper rifle is the best way to hit it without wasting tons of ammo.
Oh and as a side note, look at e picture spinner put up. You see the second head on the right? Shooting that will count as a headshot. Shooting that weird mouth thing on the left, does not count as a headshot.