Geddon does a fairly massive amount of damage just from his autoattacks, which can also crush, despite being unblockable. On top of that, you also have to deal with inferno, living bomb, and falling damage, though the latter can probably be eliminated by standing in certain places. Based on the damage I've taken from him in full FR gear, I just can't see any holy pally keeping up with that for the time it would take to kill him. Just as firelords will obliterate you despite the ease with which you can handle the earth elementals, so too will Geddon burn you to a crisp despite the fact that you can kill Garr while afk the whole fight.
We went in with a group of 4 last week after I had killed Garr. You can swim through the lava to Rag's room to avoid the bulk of the trash as described in the Garr thread, then just make a handful of pulls to Geddon and Shazzrah's room.
The actual tanking of him didn't seem too hard, as I was taking enough damage to keep my mana up most of the time. I also wore as much threat gear as I could while maintaining 365 FR. We did have a number of complications, though:
1) Positioning. We tanked him in his room (not Garr's) just outside the tunnel. This made for some tight space when avoiding inferno and keeping living bombs from blowing people up, though I suppose that clearing more flame packs would give more room. It also led to problem #2.
2) Respawns. Particularly that dog in the tunnel, which popped up when we had him at 20% or so once. Highly annoying, since they still have a huge aggro range at level 70. Again, clearing more trash would probably help here.
3) Mana drain. It wasn't a big problem for me, but it certainly hurt our healer. She kept trying to dodge the ignite by ducking in and out of sight to apply HoTs (she was a druid), but that made it difficult to cleanse her promptly. The fact that we had an enhancement shaman along made it even more interesting.
A second healer would probably make the fight a lot easier. However, the hardest part of this fight for most of us is probably convincing others to come farm the guy week after week until the bindings drop.
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