Okay, I'll try to do a text guide to help people.
1) Knife guy from toilet. Try to slash, not stab him.
2) Headshot guy on left of the partition immediately after knifing toilet man.
3) Throw flashbang to the back of the room immediately after headshotting guy on the left of partition, then crouch and sprint to the left side of chairs, shooting everyone are quickly and accurately as possible (should be 5 people lined up).
4) Run in to the next room through the right door, reload your gun, then throw a flashbang over the chair to blind the guys behind the table. You should have ~42-45 seconds on the clock.
5) Run in on the right hand side, crouch between chairs and shoot everyone who comes down the stairs (you may have to gamble on your team taking out those behind you [2 people]).
6) Reload your gun and run to the right of the stairs, thus facing the left hand door when you turn around, throw a flash into the back of the room before entry. Run in down the left channel of chairs shooting the guys on the left, middle and right of the room, shoot the guy behind the door and flash the corridor on the right (this should be your last flashbang.) Run to the counter infront of you, reload your gun once more.
7) Shoot as many people are you can, flicking between LT/L2 and RT/R2 as quickly as possible, killing around 5 people. Quickly reload once more. You should have ~12-16 seconds on the clock.
8) RUN! Shooting the person behind the chair on the rear left-most seat (through the seat from the hip).
9) As you approach the corner shoot everyone you can, don't waste too much ammo as there are 3 or 4 people here, though you CAN knife the last guy.
10) RUN! RUN! You have seconds left. Sprint towards the door and the hostage taker kicks the doors open. Shoot him to defeat the level.
Note: You must shoot him in the head in Veteran mode. In any other mode you can just shoot his leg (I tried this on Veteran
).