There are no more missions, no more preparations. The Reapers know about the Crucible and the Catalyst, and they will do everything in their power to stop you from utilizing them. Admiral Hackett will board the Normandy and deliver a speech in preparation of the upcoming battle. Remember that if you romanced Liara, you can speak with her again to view a second romance scene right before the battle. Speak with Anderson as well, via Vi-Com in the War Room. You will learn that you must reach the Ditadel Beam on the ground. A series of Hades Cannons, giant Reaper guns, are guarding London. You will have to take one team of troopers to destroy the Cannons, while another goes around to the Citadel Beam. En route to Earth, Joker will list all of the forces that you were able to unite under you. Obviously, the more, the better.
On the way to land on Earth, Joker will have to fly past numerous Reapers lingering in the atmosphere. If your Galactic Readiness ratings weren’t good enough (specifically, the Effective Military Score section) your ships will begin to fall to the Reapers at an alarming rate. Joker will warp out of the area and head straight for Earth during the scuffle. You will leave in a shuttle, but you will be shot down near the Hades Cannons. It is now your job to disable the Cannons, since you landed so close.
You will have to fight as soon as you leave the safety of your dropship. The enemy will have the advantage of height over you, having taken up positions at the top of a large staircase strewn with rubble. There will be tons of enemies awaiting you in this first battle, so use every advantage you have. Take cover behind parts of the fallen building. If you have either the Cryo Ammo or Cryo Blast power, use it to temporarilty freeze any enemies that might end up flanking around you while you either kill them or deal with the rest of the welcoming party. Similarly, Infiltrators can use Tactical Cloak to temporarily elude the enemy’s attentions.
If you are playing as one of the stronger classes, such as the Sentinel, Soldier, or Vanguard, you can actually use an intelligent combination of Biotics and weaponry to forge ahead and carve a path through the enemies to the top of the stairs. Immediately switch to Cryo Ammo if you can, and send Combat Drones or Decoys out ahead to distract the enemies as you make your advance. Toss Grenades or use Singularity combined with Warp when enemies bunch together, and equip your Shotgun or Assault Rifle as you climb the stairs. Vanguards can use Biotic Charge to clear the distance between them and an enemy with alarming quickness, so use this to scale to the top of the stairs, rendering the enemy’s height advantage moot. Sentinels should use Tech Armor to weather the brunt of the enemy’s attacks, and detonate it if you find yourself surrounded. Power your way to the top of the stairs. Soldiers can use a similar strategy to reach the top of this arena. Cast Adrenaline, which can be especially useful if you have specced it to give you a shield boost while activated, and blast your way through the killzone on the stairs to the relative safety of the top of the area. Once you reach the top, take cover and systematically kill off all of the enemies here. Then, look down the stairs and cover your squad from any remaining enemies while they rush up to join you.
Alternately, if you aren’t playing as a class well-suited to close range combat and all of the damage that will bring, you can fight from the cover at the bottom of the stairs. This will take a longer time, but it also might be the safest option, especially if you brought a good Sniper Rifle. In order for this to work, you will have to take cover behind the scrap of concrete to the left side of the battlefield. There is minimal cover at best on the right-hand side, and you won’t last long at all over there. Hide here, and get another of your squad-mates beside you to cover you in case any enemies decide to rush around the right side while you are reloading or something like that. You can either command your other squad-mate to stay near you, or have them push ahead to the first downed pillar on the stairs, to the right side of the steps. This second strategy will work best with someone like James, who is a hardened soldier with high defense stats.
Stay behind the concrete as the frontline of the enemy’s forces push forward, trying to get around to the right and finish you off. Take out this initial group of attackers with an Assault Rifle or a Shotgun, or something else that will deal decent damage at close range, like the Scorpion. Speaking of the Scorpion, this nifty pistol can also be great for setting traps in this initial encounter. Blast a few rounds onto the steps, and just watch as the Reapers step over them as the bombs detonate. You can also used charged melee attacks to kill off the first wave of attackers as they sweep around your cover, although be sure not to venture too far out from the safety of the concrete slab when attempting this. Use Biotics, like Pull or Stasis, to draw the enemies in and leave them defenseless.
As for the remaining enemies on the top of the stairs, you can either snipe them or wait for them to rush you. Pull out your Sniper Rifle if you choose the former option, and pop out carefully from cover to take potshots. Use powers like Singularity, Pull, or Stasis where necessary to line up better shots. If you catch any enemies coming down the stairs, have your party members use Grenade to take them out if they’re grouped together, or Concussive Shot if they are alone or in a group of only two. Ideally, if James is positioned on the fallen pillar on the stairs, he will be able to cover your back while you snipe the remaining enemies.
If you choose to let the enemies come to you, remember that you only have limited ammo and the enemy numbers are great. Use Biotics and Tech to take them out if you can. Engineers can set up a turret right outside of their cover, which can be absolutely integral to winning this battle. Remember to also use Combat Drones and Decoys to distract them when they come charging down at you. Using a combination of Biotics, Tech, and gunfire, you should be able to win this initial battle and soldier on to the next area.