It's been a while, but i think that you should check reactor room before Mastaba, or not?
Anyway, i think you don't need laser really, i don't remember shooting much there. Alien will block your way so you need to get rid of it. Since you can't kill him yet, you have to make it fall. so the only solution is hand-to-hand combat, because that way you can make it fall behind and eventually to fall from the laser bridge. Other solution is to drag it to the bridge and shut it off but you need to be extra fast, i couldn't do it that way at least, so i had to fight.
Hope this helps
