Iron Passage
The Iron Passage is a challenge room like Cave of the Dead in the Crown of the Sunken King DLC. This level is accessed behind the locked door just under The Foyer bonfire. Take the elevator down and open the door with the Tower Key.
Once arriving at the bottom, light the bonfire, and prepare to fight an onslaught of foes on two entirely different paths.
Primary Path
The main path is split into three major sections, each with a different set of enemies to fight.
The first area is fairly straightforward. There are skeleton knights and casters standing on a second tier waiting to attack. The simplest solution is to aggro enemies little by little, moving out of range of the Fume Sorcerer and Hollow Mage. Once the skeleton knights are defeated, move forward to the next area while ignoring the casters.
The second area is much the same, only this time instead of magic enemies, there are Possessed Armors and Hollow Mages waiting on a ledge with a group of skeleton knights. Use the same strategy as before, aggro out of range, defeat, run through.
The third area is the most challenging. In a massive room, there waits two iron golems, one which is red, and two Hollow Mages, one which is also red, along with a Possessed Armor waiting to snipe from a high ledge. The first enemy to get rid of is the regular Hollow Mage while someone distracts the iron golems. Next get rid of the red Hollow Mage and dispatch the Possessed Armor. After that, deal with the iron golems (or run away if possible), to go through the fog gate to fight the Magical Smelter Demon.
Secondary Path
The Second Path is the shortcut through the Iron Passage, which makes it entirely possible for one person to beat this stage without summoning phantoms or even fighting any enemies along the way, but speed is required.
At the beginning of the Iron Passage, there is a switch that opens a gate up ahead. This is the first shortcut. When the lever is pulled, the gate will rise up and slowly go back down. In the meantime, a Salamander statue will shoot fireballs continuously as skeleton knights come out of the passages in the walls. Stick to the right to avoid fireballs and roll to avoid damage and confrontation. If successful, you will find yourself in the first hidden room, which branches off two ways. On the left, there is a path to the ledge of the first area on the Primary Path. Deal with the Hollow Mage in the hidden room first; after that, take care of the Hollow Mage on the first area ledge; and then eliminate the Fume Sorcerer. This will allow you to collect the item on this ledge.
After this part, go to the second hidden room, where a switch can be found near the corner. BEWARE: There is a Fume Sorcerer that will appear at the very second the lever is pulled to perform a backstab. Aggro the enemy by walking into the corner to make it appear, then dispatch the Fume Sorcerer. Once the lever is pulled, the gate ahead will go up and slowly descend while a Salamander statue shoots fireballs and skeleton knights come out of the walls. This shortcut ends in the second hidden room with two paths. The path on the left leads to a crossing platform in the second area of the Primary Path, which is occupied by a Hollow Mage. Get rid of this enemy and use the ledge to kill the Possessed Armors from afar and collect the other items on the ledge they were occupying.
Going straight ahead from the second hidden room will lead to the final room on the high ledge, where a Possessed Armor can be found. Since the fog gate to the Magical Smelter Demon is in this room, just dodge the enemies and jump down to quickly enter the fog gate. This attempt can be all the more successful with an Alluring Skull or better yet the Yearn Sorcery to distract the enemies long enough to get to the door. Or the Possessed Armor could be eliminated while the Hollow Mages can be sniped from above (keep in mind, they can and will fire back with Lightning Spears). This should be enough to guarantee entry into the fog gate to fight the Magical Smelter Demon.
After the Magical Smelter Demon has been defeated, the Mask of Pharros can be acquired in the final passage.