It's possible to have it all, but the game doesn't exactly tell you how to do that. You can choose to go to save the U.N. Council or confront Viktor, but you'll only have 10 minutes to complete the task before failing the task you didn't go to first. Go to save the U.N. Security Council first, because you'll be able to disarm Viktor's detonator at the beginning of your encounter with him.

Head to the U.N. VIP area first. This is the hardest stealth area in the entire game.

Slip past the first two guards you see and a turret on your way to the waypoint. The next area is where stealth can get difficult.

Head inside the vent at the ground level just before you see the stairs and just past the first turret. This will take you past most of the guards.

Head through the room you wind up in from the vent. You'll see a hackable door directly across the hall. Wait until no guards are nearby before you start the hacking process. Enter the room.

Once you're in, the room will have a vent under the desk in the middle of the floor. Head inside and take the vent to another room.

When you reach the room that the vent leads to, head through the door that leads to a hallway with one guard — not the main hallway that has six guards. Wait until that one guard passes before you go, or just take him out. Head left to the locked door.

Use the security card you received from knocking out all 11 guards in Securing the Convention Centre, and head into the storage closet. Shut the door behind you so nobody can see what you're up to.

The storage closet has a few movable boxes and several trip mines. Pick up a box and set them off. Be sure you're far enough away so you don't get killed.

The explosion will send the guards into search mode, but you're so hidden away that they'll never find you. Head through the door, and you're now at the back of the elevator shaft that you're looking for.

Jump down the elevator shaft to the elevator car using Icarus Landing to not take any damage from the fall. Then press the elevator button for the VIP Room.

Depending on how long that process took, you may need to book it back to where you came from and confront Viktor. That means going through the same hellish area you just got through. Fortunately, you can just follow the same path you used to get here undetected.

Jump out of the top of the elevator car to get behind the elevator shaft where you dropped down, and then retrace your steps. Be sure to hustle. You'll get the good ending by making it back to Viktor before running out of time. (If you don't make it back to Viktor in time, he'll blow up buildings. Inversely, if you don't make it back to the U.N. people in time, they die.)

If you made it to Viktor in time, there will be a brief dialog. In the middle of that dialog, you'll be able to jam his bomb remote with the jammer that you received from an earlier mission, rendering it useless. Then he'll be out for blood.

The fight starts with him missing you with his cannon. Then he doesn't know where you are. There are two drones, a turret and two walking robots also gunning for you. This section looks like it's going to be much harder than it is. He doesn't know where you are, so use that to your advantage. Hit him with an EMP grenade or EMP mine, and then hit him with a stealth attack when he's dazed. That's all it takes. You don't even need to worry about the other enemies around him. That's how you get the best ending.