The Coalgut encounter is largely a DPS race.

Unstable Furnace – Avoid AoE, kite boss
Burning Lung – Stack on targeted player
Flame Circle – Avoid ground effects

Unstable Furnace – Wide-range AoE (red circle pulsing from boss), afterwards boss becomes enraged for about 30 seconds
Burning Lung – Meteor-type ability targeting a random player (other than the tank), dealing 14k damage which is divided amongst nearby players, debuff must be cleansed
Flame Circle – AoE ground effect (fiery circle)

Coalgut patrols the area and the tank should take him at the edge of the encounter area.

The boss will cast Unstable Furnace, which begins with a wide-range AoE (red circle pulsing from boss) and the emote “Coalgut looks unstable.”  Everyone needs to move out of this effect, except the tank assuming they have enough mitigation.  Next Coalgut will become enraged for about 30 seconds, doing increased damage and moving with increased speed.  The tank will need to kite him, mounting up if necessary.

There are blue crystals that spawn in random spots in the area.  Killing these crystals and kiting the boss through them will cause the boss to be slowed or frozen for about 15 seconds, and will drop off stacks of his Enraged buff (although not completely remove it).  Have an assigned DPS attack the crystals down to about 20% health, and then any ranged DPS kill it once the boss enrages so the tank can kite the boss through it. 

In addition to this mechanic, Coalgut will cast Burning Lung on a random player (other than the tank).  This meteor-type ability will deal 14k damage to the targeted player unless other players stack and share the damage.  The debuff msut be cleansed.

Finally, there will also be Flame Circle AoE ground effects, which can simply be avoided. 

References

http://rift.zam.com/en/npc/19912007/Coalgut