After finishing The Long Walk phase of the suicide mission, pick Shepards’s final squad (make sure everybody accompanying Shepard is loyal), leaving everybody else behind as a rearguard, freeing Shepard to continue on towards their destination.
When you regain control, you will be on a floating platform, complete with its own cover to play with, so you can probably guess what is coming. Not long after you have your bearings, you are back in the action. Another floating platform will come in and halt to the right of your current position. Kill the three Drones here, and a second platform will arrive at the front side of your platform. Defeat the Drones and an Assassin on this one and subsequently a third platform will fly in and stop to the left. This will have a pair of Drones and Harbinger aboard. Kill them and move to the platform in front of you.
Another, larger platform will stop to the front of the other platforms and bring Harbinger, a Collector Assassin and several Drones along for the ride. Killing the baddies populating this area will trigger yet another platform to dock on the front side of the area once again. This one has a group of Abominations and a pair of Scions on it. Eliminate these and then take their place on the last platform. Use the console here to activate the platform.
After riding the platform into the next area, during a short scene, you will be able to see the final boss. Above it are four tubes filled with some form of glowing red death fluid. After taking in the environment, get ready to fight once more! Another platform filled with Collector Assassins and Harbinger will attach to the right of the main platform. Fight these enemies off and once they are all dead, you will see the tubes above the bad guy turn red. At this point, you can take free aim and attack one of the glowing red tubes until it shatters.
After blowing up the first tube, a second platform full of Drones and Guardians will attach to the left. Kill them, then destroy another tube. A third platform will come in from the right with Drones and Harbinger. Eliminate them and then pop a third tube. A fourth platform will fly in from the left; kill the Drones piloting this one and destroy the fourth and final injection tube to watch a scene.
Boss - Reaper-Human Larva
After a lengthy conversation and a big decision or two, the big boy you just dropped will return for a boss fight.
The final confrontation of Mass Effect 2 is none other than the giant monstrosity you just dropped during the previous combat encounter. Fortunately, despite his size, this guy is fairly easy.
The Reaper’s attacks consist of a missile volley he launches from his mouth and a slam move in which he brings down his hand on you if you get too close. Both of these can do a massive amount of damage, but if you stay in cover he will be less likely to hit you. Unfortunately he also has Harbinger and his army of Collectors to fight for him. So you must be vigilant and take them out as soon as possible between attacks on the boss.
The Reaper’s weak spots are easily identified by the red glowing crystals. These are located in two major areas on the boss: there are several on his head taking the place of his eyes, and a single large crystal where his heart would be.
Most of the time, the weak spot on his chest will be hidden, either by the platform or by the boss’s tail. As such, you’ll want to focus on doing as much damage as possible to his eyes (although this is a little difficult as it moves around quite a bit). Ensure that you kill any Collectors that show up and between firefights with them , focus on dishing out damage to the boss’s weak spots. Keep plugging away with the same tactics and after a few minutes of fighting the boss will fall. The hardest part is doing enough damage while being bombarded by Collectors who can do significantly more damage than the boss itself.
ME2: Save humanity throughout the galaxy from certain annihilation
No One Left Behind
ME2: Keep your team alive through the suicide mission
After a lengthy cinematic and another brief conversation with the Illusive Man, the credits will roll and you will have finished Mass Effect 2. Congratulations!
You can now mop up any remaining missions/assignments, or move on to Mass Effect 3.