This'll help you get past the slasher.
Guide to Get the Gun
First off You'll need to go and Find the Pliers.
Head to the Left after exiting the Vent. and enter the door on the right.
After you head through the Doors go through the lasers, They look deadly but they're Just sensors.
The enter the door at the end and grab the Pliers off the Desk.
Disarm the mine in the same room, You'll need it to kill the slasher waiting for you.
Then once you kill the slasher i suggest saving, Once you have done that Head to the Corridor Near the Room you just left and Disarm both mines, You can use one to kill the Stumbling Enemy in the other room.
And finally head to the room with the gun and Place the mine at the door then Press the button and you've got it!