Gehennas

Gehennas uses Shadow Bolt, Rain of Fire, and the Curse of Gehennas. Rain of Fire is an AOE attack which has a 40 yard range and a 10 yard radius. It does 925 to 1075 of fire damage every two seconds, and lasts six seconds. Everyone who comes under attack from this should immediately move out of the affected area, other than the MT. Healers should be able to keep him alive during this attack with no trouble.

The Curse of Gehennas lasts five minutes if not dispelled, and reduces healing effects by 75 percent. It has a 45 yard radius. It must be dispelled very quickly. Gehennas' Shadow Bolt has a 3-6 second cooldown and does 2250 to 2750 points of shadow damage. He will first target the top person on his threat list. His second attack targets a random player. Have Priests cast Shadow of Protection on everyone just prior to this fight, otherwise a caster who gets hit with this bolt will almost certainly die.

You need to clear the entire area around Gehennas and then pull the guards to the south west and tank Gehennas in the center of the room. Make sure Gehennas is far enough away that the only people in range of his curse, shadow bolt, and rain of fire are the main tank and his healers/decursers. The two guards need to be tanked far away from Gehennas and need to be kept a decent distance apart, otherwise the tanks and melee DPS will end up stun locked the entire battle from the two cleaves. The MT on Gehennas will need a dedicated decurser. A mage is usually assigned to this job, but a druid could also be used. The tank must have the curse removed as soon as possible. Gehennas deals a fair amount of damage and you will not be able to keep the tank up unless he's receiving extensive healing. Once the two guards are dead the entire raid should focus on killing Gehennas.

Mages and druids in this fight should be sure to adjust their decursing priority list to place the MT at the top, followed by other melee players, as the curse greatly reduces the amount of healing a player can receive.

The Bosses:
Lucifron
Magmadar
Gehennas
Garr
Baron Geddon
Shazzrah
Golelmagg the Incinerator
Sulfuron Harbinger
Majordomo Executus
Ragnaros