Molten Constructs is the first boss encounter in Hell Fallen instance.
Iron Catastrophe is the first and is active right away. All the bosses have a frontal attack called Magmatic Quake that can be avoided by getting out of the marked area.
Molten Xenolith becomes active at around 25 seconds into the fight and starts to head to Iron Catastrophe. He spawns from the door at the far end of the room. When Molten Xenolith gets to the group (4-5 seconds), Iron Catastrophe will cast Lava Eruption and will be immune to damage. Move to the other side of the room to avoid the Lava Eruption.
As quickly as possible kill Iron Catastrophe as Primitive Melt will spawn around 1:30 into the fight. The priority is killing Iron Catastrphe because at about 1:45, Molten Xenolith will do his Lava Eruption again. Then swap back and forth until the remaining two are dead. This fight is fairly simple as long as you stay out of danger areas and focus the correct boss.