Ok so I've opened the door and I keep getting shot no matter how hard I try to shoot him first. Is there a trick here?

yes. I have the same problem and I have tried for hours please somebody help?

yer i found this a problem but 3rd time i got him,
walk slowley when near that room dont make sudden movements and dont run just walk to the door looking as straight as you can and open the door, this way he will not have his gun out, you only get one shot so fire as soon as you can dont waste time positioning for a head shot just fire, if you miss you are dead but do the same next time and you will get him, you dont need to pick anything up as you dont trade from here on.