There is a total of seven armed enemies inside the Smelting Chamber and there's another major difficulty connected with them. If you turn on Detective Mode, you should note that some of the inmates are wearing proximity mines #1. Therefore, you can't approach the from behind and instead have to focus on attacking from above #2, grates, inverted takedowns and each other known tactic.
It's not the end of trouble however, as after you let the enemies know of your presence, they will plant mines on their patrol routes from time to time #1. You can't destroy the mines using Batarangs, you would need to have the Disruptor with the Mine Detonator #2 (you can get this upgrade for finishing the Remote Hideout side mission). If you have already obtained it, you should use it after an enemy approaches one of the mines, as you're limited to two uses per location. Otherwise you will have to avoid the mines, they're clearly visible in the Detective Mode.
Try to take out the enemies with mines first, so that they don't plant too much of them #1. Only after taking care of them should you move to the other enemies, of course keeping all the necessary precautions. After you're done with everyone, use the eastern door #2 leading to the Assembly Line.
You have to be very careful here, as there's a sniper inside the adjacent room #1. Therefore you should crouch for the whole time (right trigger) and don' try jumping through the windows. Approach the door leading to the room with the sniper, look up and locate the balcony that you will be able to reach using the Grapnel Gun #2.
Turn to the south and look for the small assembly line that you can jump down onto #1. Afterwards wait for the barrier to open #2, letting you reach the room with the sniper. It would be good to have the right trigger pressed here as well, thanks to which Batman won't get noticed prematurely.
Carefully move east along the assembly line #1 and afterwards turn north. Don't go down from the assembly line too soon, as there's lots of mines in the area and one of them is beside the sniper #2.
I'd suggest going down from the line after getting directly behind the sniper #1. Carefully approach and take him down. Afterwards you can turn north and use the Grapnel Gun to reach the upper ledge #2.
After getting there, it would be good to speak with Harley Quinn #1 to receive the Hot and Cold side mission. Jump onto the ledge on the right and head north, eventually reaching a small room on the assembly line #2.
Use the northern door to reach the Loading Bay #1. Head east, stopping after reaching the main room marked by the game #2. Be careful here, as there are a few armed enemies in the area.
First off you should deal with the two snipers on the upper level, inside the previously visited Joker's office. Go slightly to the east and stand beside the stairs, from where you will see the laser beams #1. Note that you can use the Grapnel Gun to grab the ledge and end up right beside the snipers #2.
Don't try climbing up, instead press Y to eliminate both of them #1. It will of course alert the enemies downstairs, so quickly climb up and wait for the situation to settle down #2.
The three others enemies are best eliminates from above #1, though you can also use your creativity and for example use the Disruptor on their guns. Afterwards use the Grapnel Gun to return to the upper room #2, the one previously occupied by snipers.
Head west and soon you will reach a new door #1. After you reach the new location, a cutscene will start, during which Joker will challenge Batman for a fistfight #2.