After you have killed all of the enemies in the alley and looted the Med Kits and the Datapad, head farther down the alley. When you reach the end of the path, two more Brutes will jump out and attack you from the right. Be ready for them. Roll away from them and sprint back towards the beginning of the alley to give yourself some room to breathe. Turn around and, with your Ammo Powers activated, open fire upon the Brutes. Order your squad to focus on one at a time. With all of the weapons fire concentrated together, you should at the very least be able to kill the first Brute before it crosses the length of the alley to reach you. Alternately, classes that specialize in close-range combat like the Vanguard or Sentinel can use Shotguns and their Biotics and Tech to come out of this situation alive. If you have the Spike Thrower Shotgun, hit the enemy with Biotics and order your squad to attack them before beginning to charge up a shot. Hit the Brutes with fully charged Spike Thrower rounds to carve off massive chunks of health.
Head to the right at the end of the alley and run through the door. You have finally reached the missile battery. You will now be able to attack the Reaper standing over the Citadel Beam. Of course, it’s never quite as simple as it should be, and the missile battery is currently controlled by Reaper forces. You will find yourself battling through a three-way intersection, with cars and concrete strewn about in every direction making for great cover. A building to the lower left of the map is full of shelves and ammo, and makes for a great spot to take cover during the upcoming battle.
If you head to the store, you will find a Hydra Missile Launcher sitting on one of the shelves behind the counter. DO NOT use it yet, but make note of it for an upcoming encounter. Take cover in the window of the building, and have your squad cover the other side of the store. When you see enemies grouping together, use Grenade or a combination of Singularity and Warp to kill them all at once. If you can’t get a bead on an enemy, Pull them out of cover or use Singularity to yank them helplessly into the air. Use a Sniper Rifle to kill off the enemies farthest away before switching to an Assault Rifle or SMG to attack those who are closer to the window.
If you didn’t head to the storefront or if you aren’t good at sniping, don’t worry. There is plenty of suitable cover in the center of the arena, too, including the missile batteries themselves. Let your squad get your back and transition from cover point to cover point. If you can, leave your squad at the back of the arena and try to circle around the arena through the storefronts to the right. This will allow you to get behind the enemies, while your squad distracts them from the front. Hit them from behind and wipe through the center of the square to meet back up with your squad.
When all of the enemies are dead, approach the missile battery in the middle of the area. You will have to bypass the controls so that EDI can sync up with the targeting systems, thereby allowing you to fire homing missiles at the Reaper. You will have to defend the missile battery while it prepares to fire its payload. This is, of course, easier said than done, as enemies will begin to overrun your position from all angles. The first attack will come from the left. You will receive a message over the radio from Hammer Squad warning you that they have all but fallen to the Reapers, and that your left flank is now exposed. Take cover and watch the balconies above for signs of enemy activity.
You can take the Reapers head on if you are playing as one of the stronger classes, but note that even with the extra strength of a Soldier or Vanguard this approach can be deadly. You will have to time your attacks and command your squad very carefully. Position your squad so that they are spread out across the left-hand intersection. Keep one soldier to the left, one to the right, and one in the center. You will probably want to take up the position to the left yourself, so that if things get hairy you can escape through the storefront and emerge back into the streets, where you will have a little more breathing room. At first, just a few Marauders will run out on the balconies ahead. Soon after, though, two Banshees will come sauntering out behind them. Immediately order your squad to focus their fire on one of the Banshees. If you have anything that can disable shields, use it as fast as you can. When the Banshees reach the street level, chaos will unfold. It is very likely that at least one of your squad-mates will fall to the deadly touch of the Banshees. As soon as you see their shields drop, hit them with every armor-damaging power you can think of. Warp, Incinerate, Incendiary Ammo, Warp Ammo, and Grenade will all do decent damage. You can even set Proximity Mines around if you have Garrus in your party, in hopes of either damaging the Banshees or catching one of the lesser enemies in the blast. Soldiers should use Adrenaline to get in as many headshots as possible.
If you are playing as a weaker class, you will want to hang back in the streets by the missile battery and snipe. Try to hit the Banshees first, but be sure not to fire if it seems like they are going to teleport as you will likely miss. The Banshees typically teleport three times before they stop and stand for a while, so hit them before and after they perform this sequence. Hit them with Biotics and Tech when they get close enough, and position your squad to either side of you to distract the deadly enemies.
Either way, when both of the Banshees are dead, mopping up the Marauders and Cannibals by the base of the ledge ahead shouldn’t be too tough. After this wave is done with, the missile battery will fire its payload at the Reaper. Suddenly, the missiles will begin to swerve and swim before being diverted entirely and smashing into the ground. It seems that the energy aura from the Citadel Beam has disrupted the tracking systems on the missiles. You will need to draw the Reaper closer to target it successfully.