After alot of shouting, nearly breaking a controller and lots of tears of frustration I have finally finished the Marine Campaign on Nightmare.
If you just use these saves it will unlock:
Stay Frosty - Marine on Hard
I Love the Corps - Marine on Nightmare
I admire its purity - Alien on Hard
Magnificent, Isn't It? - Alien on Nightmare
It Ain't No Man - Predator on Hard
One Ugly Mother - Predator on Nightmare
Game over man - Beat all 3 campaigns
All these unlock in order, you wont get the individual mission achievements so play through campaign before using these.
Just beat one mission on easy and all the above will unlock, thanks to mikehunt1822005 for finding this easy way to get em to pop.
It will also unlock 10 spear kills, 10 headshot kills and 30 pistol kills if you get one kill with each of the required weapons.
Dont forget to leave thanks and rep.