With Xenia out of the way, you can now concentrate on Trevalyan aka Janus. But you still need to go through this complexity before you finally confront him.
The Tunnel of The Dead Overkill[edit]
You start out in the building. Again, stealth or shoot. The notable locations is station areas, security cameras, patrolling guards and turrets. So plan your way through until you reach a very open and epic area.
Starting Up, Going Down[edit]
You really need to tread carefully. This large area will be very hard to traverse if you're not careful doing stealth. Look for covers like behind the wall example. Take your time, picking enemies one by one unless you're pushed into a gun battle. Don't forget to take the bombs along the way or you could easily fail the mission.
Momentum Shifting[edit]
Go through and put those bombs in those areas as they're yet ready to explode. You should be inside this sort of octogonal area. Take out security cameras and enemies until you reach the elevator. Yet again while you wait for the elevator, take care of the remaining guys until the elevator's ready.
The Last Stretch[edit]
There's one more obstacle in the way. You have to go through the open area which literally put you in danger and can take many tries to get through. So shoot your way through and take care of trucks, rocket people and soldiers that can leave you vulnerable. Take care as many as you can and shoot them before they shoot you. Once you reach the stairs, get ready for the major lengthy finale.