When corporate employees escaping a building blaze are machine-gunned down at the fire exit, a PR girl becomes trapped in a stairwell, and must outwit a deranged gunman.

I like this idea but there a few problems with your logline itself.

Firstly, try and link everything as in: Why is the gunman targeting this building? Is it a bank that foreclosed on him?

Secondly, your protagonist (not your situation) should be introduced first, and why should we sympathize with her? What's her flaw?

I like this idea but there a few problems with your logline itself.

Firstly, try and link everything as in: Why is the gunman targeting this building? Is it a bank that foreclosed on him?

Secondly, your protagonist (not your situation) should be introduced first, and why should we sympathize with her? What's her flaw?

The wording of the logline is working against it. Replace corporate employees with the MC description, also I suggest you describe her with anything else other than girl. Is she a teenager or an adult?
I presume an adult, therefore best to describe her accordingly - PR expert or PR manager.

Here is my try:
After a gunman sets her work place on fire, a PR manager trapped in the stairwell must ?outwit the psychopath to stay alive.

As Skpough1 said, what does outwit mean? How will she, and the audience to that matter, know beyond a doubt that she has succeeded and found safety? What specifically is her goal?

Lastly, is this a short or feature length film? I get that you're going for a high paced slasher, but I don't think there is enough in this concept for a full length feature.? I could be wrong only the plot seems rather thin from the logline, I suggest increasing her obstacle.
What if the gunman had a few psychotic mates working with him? This way she has to fight a bunch of bad guys, John McClane style, instead of just one.