Mass Effect 3 - Priority: Cerberus HQ

When you gain control, you'll be attacked by a group of three Cerberus relatively close. Take them out quickly and then an Atlas Mech will appear. Focus all of your attention on the Mech. Keep overloading and switch weapons when appropriate. Once the Mech is down, watch out for the Nemesis on the balcony up and to the left of you. Try and time your abilities appropriately. You may have to aim for the lock on to work. She always pops up just after she moves so wait for her to move and then spring an Overload and Singularity on her. Then just clear the enemies in the Hangar. It should only be a few Troopers and Centurions left.

Go up to where the Nemesis was and open the door there. Just inside is a Console to deactivate the Vent system. Turn it off and then fight the Cerberus coming into the room. There will be a Guardian among them. Kill the three of them and move out of the room from the way they came. When you get part way across the bridge, more troops will come at you. Watch for grenades and deal with them. Follow the bridge to the next room to activate the Fighter Controls. Go through the next couple of rooms until you come to a store room with ammo and a PDA. Go down into the Hangar proper and use the Clamp Release terminal to blow up the door.

An Atlas Mech will come through the hole in the wall and some Troopers will attack from the walkway above the hole. Since the Atlas will likely be covered in smoke, take out the Troopers first and then focus on the Mech. When you destroy this one, another will appear from a door toward the back left. Destroy this one too and then enter the hole. Turn left and approach the bypassable door.

Drop down the ladder to your left and you'll come to some tunnels with Troopers. Kill the troopers and move on to the next group. In this group will be a Nemesis and a few Centurions. Keep back and take your time. When the Nemesis is hiding, take out the Troopers and Centurions. When the Nemesis appears, try and take her out so it's one less thing to deal with. After you clear the area, check near where the Nemesis was for a Med Station, Salvage, and Ammo. Then climb the ladder to the Cerberus Labs.

In the labs, listen to the files and then work your way through the fires.

When you go down the ramp, you'll come to a tougher fight. There will be quite a few Centurions and Engineers in here along with their turrets. Also watch out for the Shield Generators in the area. Once you get a foot hold, it shouldn't be too difficult to slowly move forward. In this area, enemies will continually respawn so you'll need to keep pushing forward a little at a time. After you get past the first group, you'll come to another group and EDI will say that the generators for the barriers are to the right. At this point, go far right and you can work up the enemies left flank (their left, your right). Once you get past them, you'll come to one last battle where enemies will come down from a ladder at the back. Clear a group of them and move past the first set of barriers and go right. You can slide around to a small space between the boxes and the walls that will bring you alongside the ladder. From here you can kill the enemies and when they are dead, they will stay dead. Climb the ladder.

Make your way through the labs, grabbing the PDA on the way. Exit the room and turn right. You'll see a bright blue light. Run towards it to jump down. You'll come to a large dead Reaper. On these walkways will be another fight. You'll come to a Nemeses, Phantoms, and some Centurions. There are also a few shield generators around here. The catwalk makes a large U shape and you can see one of the generators at the third arm of the U. Take that out immediately and then try push resistance back on the first arm until you can get a good angle to take out the Shield Pylon at the corner. Take that out and it'll ease up. Take it slow however because you will be attacked by Phantoms frequently and you need to have some room to take them out. If you see one, quickly back track to a safer spot and try to hit it with Overload and Singularity at the same time. Watch for the Nemesis at each of the corners. Once you make it to the end, climb the ladder.

When you get to the top of the ladders, you'll come to another long catwalk and be attacked by a similar group of enemies except that they'll all appear in front of you. This will make the fight a lot easier. Just watch for the Shield Generator in the back left corner. Climb the ladder at the end and then make your way through the next room. Next to a console in the middle of the room are a PDA, a Med Kit, and some ammo. Collect these and activate the log. Go through the door at the end and up the ramp.

In that room will be the boss fight. Kai Leng will come at you and try to attack. You'll have to knock his shields down some and then he'll summon some Cerberus Troopers. Kill they quickly and then set about with Kai Leng again. He'll go about summoning allies in intervals until he starts summoning Nemeses and Phantoms as well. When he summons help, you'll have to kill them all before you can damage him again. After four waves of help, you'll finally be able to kill him. Killing him is quite easy actually, it's his help that's difficult.

Once he's dead, you'll have a few interrupt opportunities to take as you please.