Pretty easy boss for most guilds, just need some good positioning and experience to pull it off. Having your healers able to react quickly helps quite a bit here as well.

Abilities

Cleave: The adds for this fight have a cleave ability that stun whoever they hit but it isn't a big deal.

Gehennas's Curse: Gehennas casts this every 30 seconds and reduces healing by 75%. Make sure you cure this on the MT as fast as possible.

Shadow Bolt: Gehennas shadow bolts quite a bit and they can do over 2000 damage. He randomly targets someone to cast this so have healers ready to heal whoever gets hit.

Rain of Fire: Gehennas casts a AE rain of fire spell like the warlocks spell but his version does quite a bit of damage and anyone hit by this should run out as fast as possible.

Pull:

In this fight you should first clear the area of mobs, your going to need the room. Have 1 MT and 2 Off-tanks set up, also you need 2 hunters to pull.

Have you tank grab gehennas while the 2 hunters pull Gehennas's guards off to the sides.

Fight:

Gehennas has those nasty AoE spells and shadow bolts so make sure everyone is ready to move/run to avoid the rain of fire. Also if your MT is hit by the curse get this off extremely fast or he may eat dirt.

The add's are easy to beat down whereupon you need to focus fire on gehennas. Gehennas will target the rain of fire on those beating him down in melee so be ready to run out. Your MT will need to be able to reposition gehennas if he's hit by the fire AoE.

Other then that Gehennas is a easily killed boss, good luck.