Don't hang around!... I died on this boss fight TWICE and even on the easier difficulties because I underestimated it. You've got to keep moving, whilst stopping to turn and shoot occasionally. I found myself running in circles up and around the staircase surrounding him.
Also I'm pretty sure that you need to shoot him until he opens himself up, THEN he'll take damage. You'll find Plant 42 in the residence.
Take a good couple of first aid sprays with you just in case, you'll have plenty of these by this point on the lower difficulties unless you really suck, and something like the shotgun with a rook of ammo.