Act 1: C2 - Desperation
Take the side door to meet Tai, a new ally. Follow him to the ambush point, and after talking with Anya you should take cover by the window; just press A to duck, even though it's not much cover. Wait for them to get toward the middle of the room, and when they all seem to be near the explosive tanks, shoot them with a good spray to blow the whole group up; otherwise you'll be in for a much more difficult fight.
Take the left path and down the hall there is an open room on the side that has a journal. Continue to some ammo and then go into the next area. Take cover behind a column as Locust move under you. Now, if you're new to this game, or gaming in general, you should know you can take cover and then press LT to aim from behind cover. You're still hit-able, but at least you're protected a bit. Three enemies will head for a door, and then one straggler; hit as many as you can with headshots.
Once they are gone, head left and take cover in the narrow stretch, or you could anticipate the enemies opening the door and take them out from here. Either way, clear them out. Dom and Tai will be told to take the lower path as you go forward down the hallway. First you can grab the Hammerburst, a powerful gun that doesn't allow as many shots as the Lancer, but it's few shots may be slightly more accurate for you and more powerful, just depends on how you like to inflict damage. Grab it and the ammo along this narrow strip, then go down the hall.
*NOTE: Sorry if I didn't cover this properly, but this game is all about pop and fire shooting. That means you hide while the enemies attack, then you pop up for a while and either attack them as they are firing elsewhere or wait for them to pop up. You can guess where they will pop up, but that usually doesn't work well. You're best move is to let them attack someone else, then pop up and fire. Of course on normal you can take a lot of damage, so do whatever works.*
At the corner you'll be fired on by a few enemies in the room. You can blind-toss a frag by just holding it and pressing fire; it should go in the general area you're looking at. Do so and then finish off the other guy. If one guy starts to crawl away, you can go up and pick him up with the A button. He is now your meat shield and will absorb most of the damage you would take. There is one more guy on the side of this room, so hopefully you can test out the shield aspect on that dude. Once the shield is of no more use, just press X to kill him.
*NOTE: If you don't want the shield, and sometimes you may not, press X while he's on the ground to curb stomp, B for a quick kill, and Y for an extended kill. It's purely for your own amusement, so please, keep a focused mind when in the heat of battle.*
Grab the ammo around and head down the stairs to the next area. Take cover, probably on the left doorway, and kill the two enemies in here. Go in for a paper at the desk, then out the door on the right; the back doors just lead to one grenade in the dead-end hallway.
Outside is a fight with a few enemies, so take cover and fire. Then a Reaver with some mounted dudes will drop in, so you can stay behind cover and wait for them to arrive; shoot at the mounts a little to drive it away. There is a Boltok Pistol no the bench, which is basically a revolver type gun, and picking it up does replace your Snub Pistol. The Boltok also has a slight zoom function if you click the RS. Be sure to comb the area for ammo, with one box to the far left.
Proceed into the next building and down and around the corner are two Drones. Attack them through the window, then come back a bit for ammo and frags in the surgery room. Move along for a call from Anya and take the path on the left. The first door on the right will be into the cafeteria area where the enemies wait. However, you can run straight up this path to a doorway at the end and from here flank the enemies as your guys move up the main portion.
On this side is a door you can kick open for some ammo, then go back a bit for a tiny room with open doors; a letter is in the desk. Now move up the main area and at the end you'll see a chopper fall through the skylight. Two enemies then appear on the upper area, so either take cover in the middle and act as the decoy, or you go to the right and attack them from the side; or anything in between of course. There's ammo on the left on the ground if you need it.
Go up the stairs and through the door. There is a frag on the left and ammo and another paper on the right. Head down and take cover in the middle reception desk on the right side (from where you entered), and wait for the Locust on the right. Kill three, and maybe you'll be lucky enough for one to get close and crawl when beaten, then use him as a shield to handle the rest and the three that come from the other side. Kill, collect ammo, box in the left hall, and then out the door.
Fight the Drones and the riders from cover down this middle stairway. Once it's clear, go to the right and by the wall are some tags. Head down to the open and take cover so you're protected from the right. A chopper will clear up some of the enemies, then make your way up the road. To the left is a store you can jump in and from here flank the enemies. Clear them out, grab any necessary ammo, then go into the open and be ready for one more fight as you continue down the road. Clear out the last few enemies and the chapter is over.