Beat the second boss:
     You  can get the Medic to follow you in by pushing him up against
the big door then telling him to follow you and then open the door.

     You  don't  need  the  medic  just  get the grenade launcher from
behind  the  wooden  boxes  by  using a frag grenade and use the flame
grenades you will get with the launcher to burn the boss up.

Keeping Williams:
     On  the  level where you are chasing the norwegian with radio and
he  locks himself in the building, well you know how when you run into
the  other  building to get the key to the other building and Williams
turns  into  the thing? I have a solution! Instead of running into the
building  go  to the door an when Blake says "It reaks of fuel here it
must  be all over the place" throw a flare and grenades at door and it
will blow up! It will also save you alot of health. And then you go in
and  get  the  key and you get to keep williams the rest of the level.
But dont get in the building or he will change into the thing!