So I was hearing about alien bosses and such and came across an idea. so hear me out, here is the scenario.
you just pressed a switch in a room with steam blowing, red emergency lights swirling, and with shadows covered everywhere. when this switch is pressed you start to hear an emergency siren, a door opens and you see a predalien(or preatorian) just killing a marine. (the camera zooms in like in gears of war when the alien appears) over your radio you hear "omfg what is that, GET OUT OF THERE NOW!!! RUN RUN RUN"
so you turn through a corridor, you do not have the weapon layout capable of killing the alien. so all you can do is run, so you run through dark corridors steam smacking you around knowing that you got an alien on your tail. Keep in mind that it's still dark as hell. (For the consoles, you can feel your controller rumble right when the alien is on your heels from his footsteps) you must jump over obstacles and crouch under some, pipes and what not. also it could be possible to (either tap a button for consoles or hold a button for pc) to pull something infront of the alien to buy you some time. (a lever to drop a destructable door but the leaver is heavy, adds suspense when your pulling this slow damn thing and you see the alien from the corner of your eye coming at you, could be your pulling down some type of pipe/shelving/some debris, to slow him)
this could end in 2 different ways.
1. the alien gets killed by you with some environmental deathtrap (molten metal sprayed over him, some type of airlock that sucks him out, a huge fan that cuts him up, ect. ect.)
or the one that I want to happen.
2. you will see this fella again, at the end u reach someplace where he can't get you. there will be these "chase scenes" you can't kill him and you must run for your life. this would happen i guess 2-4 times during the marine campaign at the end, you can pick up a heavy weapon (lets say minigun and dice him, OR do an enviro kill which is preferable, could be a special alien for what we know)
Think Gears of War with the berserker
The Dahaka from Prince of Persia 2
There are a couple types of fear. the SURPRISE Fear, which you will get alot of and is easily pulled off and there is the suspense, thriller fear and i think that's the best way to pull it off, how i described.
I freaked out when i had the berserker on my tail. that shit is scary.
and mind you that the sprint feature for marine is established, so you would be doing that when your running, you can't sprint backwards, you try to turn around and pop him, your gone.