The Blast Furnace is the final boss encounter in the Slag Works in BRF.

This is a 3 phase encounter.  The first 2 phases are primarily dealing with adds, and the last phase you deal with the "boss" itself.

I will go over each Phase briefly as there are a ton of adds and will NOT delve deeply into each of them.

One ability to note that lasts through the entirety of the fight is Blast.  The furnace will periodically burst mild/moderate raid-wide fire damage.  This damage increases via the Heat meter.  The higer level of the Heat the higer the damage of Blast.  After you reach Phase 2, the Heat meter will stop growing, and will slowly decrease, which will cause the damge from the pulses to gradually decrease. 

Phase 1 - Sabotage

There are Heat Regulators on each side of the Furnace, you must disable these to move to the next phase.

There is a mini-boss this phase called Foreman Feldspar that needs to be nuked down immediately at the start of the phase.  He deals aoe fire dmg to the raid and spawns fire shit under randon players.  Interrupt his Pyroclasm ability to negate extra fire damage being thrown at random players.

There are 3 other adds this phase that you will deal with primarily.

Bellows Operator - these just periodically increase the heat meter of the Furnace.

Security Guard - These need to be tanked, and randomly pop up Defense shields that makes any mob within the shield immune to damage.  When they pop this up, just move any other mobs outside the shield.

Furnace Engineer - These are the primary kill targets.  They throw out Electrocution which is a "chain lightning" ability that spreads up to 2 targets within 5 yards of the initial target, so stay loosely spread.  They will also try to Repair the damage youve dealt to the furnace, this is INTERRUPTABLE!  They will also throw Bombs on random players.  Players affected with this will blow up after 15 seconds and deal dmg to anyone around them.  Players affected with this need to run to the furnace doors and explode on them to deal damage to the Furnace.  Once this add dies, he will drop a sack of bombs that will explode after a set amount of time.  Players need to CLICK on the sack, it will give them the Bomb debuff, same as the ability above, run to the Furnace door and blow it up.

Phase 2 - The Heart Exposed (occurs after Furnace Doors are destroyed)

The boss leaves the Furnace, the Heat meter will start to gradually go down.  More adds spawn.

Security Guard - same as above

Primal Elementalist - 4 of these adds will channel an ability on the boss, freezing him in place and making him immune.  However, they ALSO have an immunity shield and will take no damage.  In order to break their shield, you need to kill another add, the Slag Elemental, on top of them.  This will remove their shield so they can be damaged for a limited amount of time.  NUKE the Elementalist before the shield pops back up on them.  DO NOT USE HERO on them.

Slag Elemental - They will Fixate on a random player and will remain fixated on them until either the player is dead or it dies.  Once it dies, it will explode for ~100k fire damge to all enemies within 8 yards and go Dormant.  It will regain 20 energy for each Blast the furnace pulses out.  Once it reaches 100 energy it will re-activate.  Kill this mob on the Primal Elementalist to break their shields.  They also cast Burn which deals mild damage to current target.  Its a long cast and fairly negligible, but interrupt none-the-less.

Firecaller - These adds are a HIGH priority.  Interrupd=t their Cauterize Wounds ability, as this will heal the target for 10% of its max health.  Very bad if it goes on an Elementalist or the Slag Elemental.  Lava Burst - deals ~55k fire damage to a random target and is fairly negligible.  Better to interrupt CW compared to this.  Main reason to nuke this mob down is Erupt, where it will spawn lava to spawn underneath a dormant Slag Elemental and re-activate it immediately.  You want to avoid having multiple Slag Elementals up at the same time.  Also puts out Volatile Fire debuff, this explodes after 12 seconds dealing ~75k fire dmg to players within 8 yards.  If you get the debuff, run out.

Phase 3 - Freedom (occurs once all 4 Primal Elementalists are killed)

At this part of the fight, all that is left is the boss, and any adds that might still be alive.

The boss has 3 primary mechanics in addition to the Blast damage coming from the furnace.  2 of which are tank based, and the 3rd is just a large slag of lava on the floor.

Heat - Stacking ability that leaves a Fire DoT on the tank.  The damage that Heat deals increases by the number of stacks of Tempered the tank has.

Tempered - to break it down simply, an additional stacking debuff that increases the damage that you can take from heat.  This stack can NEVER be higher than the number of stacks of Heat the tank has.  Taunt off on low stacks of Heat to prevent this from getting too high/crazy of damage.

Melt - Heats the floor under random players turning it into a LARGE zone of lava, dealing dmg every sec. DON'T STAND IN FIRE!.  The room will gradually fill up with these, leading to soft-enrage.

TLDR/Saedo's Cliffs Notes

First phase, adds.  Tank adds, kill adds, don't stand in front of adds, don't stand in fire of adds, grab bomb or get debuffed as a bomb (via Engineers), become the bomb, run to a Heat Regulator (2 of them 1 on each side), blow up the Heat Regulator.   Interrupt adds that try to repair/heal the Heat Regulators.  Divide raid in 2 to be responsible for each one.

Second phase, 4 primal elementists must be killed but they have a shield.  Shields brought down by Slag Elemental dying and exploding near them, they random fixate on players, must kite and kill near Primal Elementists, then blow it up (melee don't be near that).   Also, raid aoe damage and other adds to deal with.  Firecallsers sends out random debuff makes you explode, so run away if you have it, interrupt healers.   Security Guards can protect their allies, so kill them, also apparently MC'able and can kill/debuff elementals if need be.

Third phase, kill the Heart of the Mountain, some massive raid wide aoe going on here.  Tank swap mechanic too.  Fire patches on the ground to not be in.   Pop Hero and go.