This room is tough. You want to wait for someone to go under the gargoyle you start under. Climb up there and wait for an inverted takedown opportunity. After you get him, start hopping around. There's no real pretty way of doing this so just wait for people to separate themselves from the group before you drop down on them. Use the fire extinguishers so you can drop down and take someone out without being noticed. You'll have to use all of your tricks to get through this room.
After you clear the room, there is a shuttered door in the Northeast. Open it and use the railings to cross to it. Go in and open the next door as well. Climb in the vent and into the next room. Go to the far end of the room and use the conveyor belt to get into the room with the guard. You can hop in one of the boxes (MGS?) and the conveyor belt will carry you around unseen. It won't take you out of the room though so ride to the end and do a silent takedown on him. Hop up on the ledge above to find Harley Quinn. Talk to her for a side mission. Then continue on behind her.
Go under the small door into the next room and then take the door on your right. In the next room, if you can, disable the weapons of the two snipers up in the doorway. Then you can just approach and take out the three guys on the ground. Clean them up and then hop up and get rid of the snipers who will try ineffectually to shoot at you. Follow the arrows on the floor.
