definitely the doctor...and the green room. The doctor because he would always do the "Force" on you and lift you up the air. i didn't know that you can shoot his hands to make him drop you untill my 5th try fighting him.
The green room because i ran out of machine gun and shotgun ammo to quickly. plust the worms got annoying blending in the background. the second time was easier- i survived with one health to spare!