yossy666

Hide in rooms, switch to shotgun, and wait for them to come in. (Do this when you're at the last few parts of the level when waves of guys come after you).

To kill the shotgun dudes, and the buggers with shields, toss grenades, and duck.

To destroy the machine gun nest at the very end... Throw grenades, RPG and other big band stuff. Bullets don't tend to work.