I haven't played but I know this from wikis
What you need to do is hold **PERFECTLY** still
don't move
don't move your camera
don't swap items or emote
it will kill you if you move
it will pass shortly
if it doesn't that means it has seen you move and it will just stand near you until you DO move so it can shoot you
if you do feel lucky and want to attack it with a gun or stopsign
hit it in the eye that sits in its mouth
that's it's weak spot
when you kill it it drops a shotgun
and one to two shells