If I were going for something that would be a challenge, almost everywhere. But to name specifics, the areas where clearing out zombies (or other enemies) to get by are essential. The hallway outside of the club key door (where you need to board up the window to stop the zombie later) as well as the library would be two spots I could think of to make things challenging.
I would put them in the respective spots where it would be canonical for them to be so poison would be in the sewers, armored would be in the nest and maybe a few scattered around the station or the sewers, and the special infected would be sprinkled everywhere due to them just being mutated infected if i'm correct
Would definitely have the armored guys at the end of Sewer and beginning of Nest. Maybe 1-2 towards the end of Nest. I don't know where I'd put the purple nurples. Probably the courtyard that Claire has to go through with the basketball hoops, bus, etc..? Regenerators, would put them in the sewers, and get rid of the G abominations. Muaha!