This is definately one of the easier levels, as there is no end of level boss fight to worry about, and at no part of the level are you "trapped" in a room.
There are three main battles in this level - firstly at the start as you work your way down the building. Take your time, dont rush, and use/recharge bullet time as you go using RB.
The next battle is quite a long one, first as you emerge from the building - again take your time here, and just approach solwly to limit the enemies you face.
The last battle is after you leave the sewer up some steps. My advise here is dont rush forward! Egde up and down the steps so you can just see heads, and pick the enemies off gradually. They do not unlimited spawn, so no concern here. Be careful towards the end as there are some rocket guys, and make sure you pick up the rocket launcher before you exit the level - its v handy at the start of the next level for the snipers at the start!