There are 7 "missions" in act 2 you need to complete in order to get this.
All are relatively short and relatively straightforward. Just take your time, use cover and don't rush in.
I found the sniper girl the best character to play as. Always use LT as the grenade shot takes out the big guys with one hit and ammo supplies also mean you never really run out. You also get to use the ghost bullet (RB) which is also a good tactic.
Once or twice you will reach a dead end with the big guys on a wall above (and once flame throwers in front).
Use jones and RB, then LB to teleport into the big guys, look at the water towers(?) and push X to destroy them. On Brandenburg Gate the tower thing to shoot is behind the big guy.
Also at the end, I was confused what to do. Select the girl with the knife and go to the back right from where you enter - it seems to then trigger a cutscene.