When playing as a marine it is really satisfying shooting the aliens but then tiresome when you have to fight wave after wave. You play as a marine who has to find his way out of a situation and there is only one way you can get out, by killing everything in sight.
Sometimes it is a good idea to just run away and leave your alien opponents behind you - it does work.
I have only played a bit on alien but completed the game on hard when being marine but predator however, remains untouched.
I must note that there was an aimbot in play when progressing through this title. I had no control over the aimbot I either turn the game off and not play or live with it. I don't know maybe it had something to do with 20th Century Fox having being part in production.
Just buy it it is so cheap now have a bit of a thrill.