After taking out the enemies and grabbing the items by the two-tiered arena, take a right at the back corner and follow the path to the next catwalk. You will find another group of geth blocking your way. Take cover at the start of the path. The geth will be grouped fairly close together in these close quarters, so powers like Grenade, Singularity, and Shockwave can be quite effective here. A quick flurry of Biotic attacks will leave the geth open to being rushed, so charge forward after your initial attack and mop up the synthetics as they try to recover.
When you reach the other side of the path, turn to the left and attack the geth on the balcony above. A Sniper Rifle or scoped SMG works well enough for this task. Otherwise, pull them out of cover with Biotics so you can get a clearer shot with an Assault Rifle. Kill the Rocket Trooper first. After the battle, grab the Assault Rifle Stability Damper IV near the right side of the entrance to this area, then head towards the exit to find the Geth Plasma Shotgun and Salvage, along with a Med Kit. Leave to the right and head through the door. Follow the path to the next area. You will be met with a shield, which you will have to defuse by hitting a switch. Go through and turn left. Hit the button to override the computer. When you do, geth will begin to pour into the area.
Legion will ask you to defend him while he hacks the security console. You will have to retreat a little, as the enemy forces can be overwhelming here. Watch out for the Pyros with their flamethrowers. Either take them out from afar, or, if this doesn’t appear to be an option, use Biotic Charge on them and take them out while they are still reeling from the attack. Otherwise, command your squad to take cover and pick off the geth as they pour in. If you venture out from cover, be careful of the Proximity Mines that the geth will have set around the arena.
Pick up the Med Kit from the right side by the elevator, then head back to the computer to override the computer. A Geth Prime and two Rocket Troopers will run in and attack you and your squad. Backpedal to cover to avoid their attacks. Focus on the Rocket Troopers first. Have your squad focus on and kill one of them while you kill the other one. With only the Geth Prime left, you and your squad can both focus on it to kill it fairly quickly. Just make sure to take down the Drones it summons, too.
Alternately, if you are playing on a higher difficulty, you might actually want to take out the Geth Prime first. This is because it will be spawning Drones during the battle, and these elusive little enemies can prove just as annoying as the Rocket Troopers since they can essentially just hover past your cover. Take cover towards the console in the back, so that you will be protected from ranged attacks and it will take a while for the geth troopers to close the distance between you. Order your squad-mates to focus their attacks on the Geth Prime, and do the same yourself. Duck back into cover each time you see the Rocket Troopers taking aim with their rockets. From cover this far back, you will be able to use your Sniper Rifle to attack the Geth Prime. It should go down soon enough with you and your squad all attacking it. Now you will be free to focus your attacks on one of the Rocket Troopers. Take them out one at a time, switching to a close to mid-range weapons if necessary.
When the battle is over, get in the elevator and take it to the next level. Get to cover and grab the Geth Spitfire. A group of three Geth Primes will take this chance you attack you. Use the Spitfire and your squad to kill them, one at a time. Stick to cover, and shoot down the Drones that the Primes will summon to seek you out. If you let the Drones get too numerous, it’ll be easy to get overwhelmed. Have your squad focus on one of the Primes, while you alternate between shooting down Drones and shooting the Geth Primes. Also, be sure to send out a few Combat Drones of you own, or a Decoy if you have EDI in your party. Keep the Geth Prime’s attention focused on as many different targets as possible. Then, use Biotics to weaken the Geth Prime’s armor and shields. If you run out of ammo in your Spitfire, and this weapon has been working well for you, note that there are two other ones scattered about the arena. The first will be to the left next to a locked door. The final Spitfire will be located on top of a crate on the right side of the level.
This battle will be a long one, but not necessarily impossible if you stick to headshots and keep throwing out Decoys and Combat Drones. When the encounter is finally over, head to the ledge on the other side of the walkway. Aim down into the pit and lock on to the red sphere in the lower center. Paint the target for the Normandy, and the ship will fly over and bomb the station. This will get the attention of a Reaper, which will emerge from the bombed area and begin attacking. Run to the left side of the area, where Legion will be waiting for you with a ship. You will have to get there fast, as the Reaper will begin to chase after you. Climb into the ship. Legion will begin to speed away, but the Reaper will give chase and charge up its laser. With a crimson blast of the laser, it will dash your escape craft to pieces. It looks like you will have no choice but to fight the Reaper somehow now.