Past the Flames
Walk around to the left and enter the bar area. After you loot it, go up the ladder.
This next area is basically just one long firefight. After you kill the rusher with the flaming bat, you just have to keep advancing to new cover and fire as you go. There's no trick to it, just don't overextend and you'll survive. Your goal is a valve on a pipe. Turn the valve to kill the fire and then ride the lift back down.
The fire blocking your path is gone now, so take out the new guard that comes out to investigate, and then advance to the big desk in the center. You'll need to take out another group of guards that have rushed in. Just stay low and peek over the desk to take them down. Once they're dead, the fires to the left will die down and you'll be able to move forward. Before you do though, go to the right to grab the Gearhead Shotgun collector card, and stop and loot all of the ATM machines by the entrance.
Your path on the left side will depend on what the AI decides to do. If they set up behind the desks close to the hole, they'll be able to put a lot of fire down on you. You'll probably want to toss a grenade to break the blockade. If they hang back instead, you should be able to just use the nearby desk for cover and pick them off.
As you move toward the door in back, a heavy should walk out to ambush you. Sprint back behind one of the walls and crouch. You should be able to stand up and fire from behind it. If you're running low on ammo, then fatboys and grenades work well. Toss a grenade at his feet and put about twelve fatboy rounds into him. He should die.
Move forward into the offices and loot the stuff off of the desks, and then approach the door. More flames will block your path, and two heavies will come in, one through each set of flames.
I suggest that you quickly kill whichever one is in front of you. Chuck a grenade through the fire and start firing fatboy rounds. If you're quick, you can kill him before he even makes it to you. Look to the other side of the office through the windows to see if the other heavy is coming. If he is, watch for which way he's going and run for cover in the "safe" office. You should then be able to chuck a grenade and flank him.
Once they're both dead, the fires will die and you'll be able to go through the next room.