Gears of War 3 - Threshold

Head straight into the building, turn the corner and at the end of the hall is a bust on a pedestal, it's a collectable. From there, turn left and enter the big foyer. At the other end, hit the elevator switch. At the top of the elevator, turn around and hit the elevator switch again. The other elevator door will open and on the wall will be a poster. It's a collectable. Walk off the lift, turn left twice and you'll end up outside by a giant fountain area. Take the stairs down and right. You'll have some theron guards to deal with on the way up the stairs. At the top are an Armored Kantus and some mortar teams so it's important you get up the stairs as quickly as possible. The Mortar will be on the part of the top floor that overlooks the rest of the fountain area.

When you get near the door, stalks will come up and one of them will bring a gunker (and drop it right in front of you). You need to eliminate the lambent here before proceeding. It's your choice whether you go for the Gunker or the stalks first. The gunker never respawned for me so it might be easier to kill him first and then the stalks. Either way, deal with the lambent and you'll be taken inside.