When I played Crushing I had so much fun in the Courtyard the first time. I killed five enemies with my five Sniper shots, took out the Grenadiers from long range with the pistol, and then just threw myself from pillar to pillar blindfiring the Shotgun at everyone who charged me. It was one of the few times on Crushing I actually used cover, most of the time it's better to stand behind a wall and just edge out a bit so you aren't exposing your whole body.
The sewer is definitely the worst bit because the pillars in that room don't act the way they should. They're circular pillars but they act square; you can only stick to cover on the 'sides', so you're always at the wrong angle. You should be able to stick to them and move all the way around them, but you can only turn Nate left or right. Actually taking cover in there is basically asking to be murdered.