Those are hints for the bosses that will help you get them the easy way!

Joanson:

Just keep running in circle(Facing him) in the room while shooting at
him. That way he cant target you and he cant get you with his serum that
the easiest way I found to beat him up. But make sure to break all the
glass walls in the room so you will be able to move through them.

McCall:

Remember the barrack you came in the level by? (first one on the left
when you open the hangar door) Get to it and get in there. Get to the
wall on the right and open the 2 window flaps. Then jump on the bench in
front of you and crouch. Then look out side (facing north,so you can see
the back of the other barrack on the right. Now wait for McCall to come
he'll come from the right so get ready to shoot as soon as he pass and
then he'll come back from the left (from behind your barrack) s shoot
him again. keep on doing this until he dies. he will shoot you a couple
of times but not enough to kill you.

Sub-Marine Captain:

When he's done delivering his speech, the door came in by will open and
two guards will get in kill them. When they are dead hide behind the
missiles and wait till the captain reload his gun and when he is
reloading attack. then hide back and you can shoot him by the space
between the missiles too.

Chopper:

Get in the boss area with the riffle and run around the crates to the
other side, grab the rockets on your way and hide behind the crates,
like in the middles of them. Wait till the chopper is done firing the
the machine gun then show up(on the left side when facing the crates)
with your rocket launcher and shoot one rocket when he stops moving then
hide back right away. just repeat the pattern till he dies. if you need
more rockets just run in the boss area there is lots everywhere.