Number 5: Magtheridon
Coming in at number 5, we zoom over to Outland and into the depths of Hellfire Citadel. Magtheridon used to rule over Outland and its various denizens. When Illidan made his escape here, he managed to subdue him. What was his purpose? To create an army of Fel Orcs of course!
A long, long time ago, when Mag was truly the only end game boss in Burning Crusade at the time, he had a bunch of these mobs called trash. They patrolled the entrance down to his central chamber. Yes, his trash sucked and it sucked a lot. I remember their Shadowbolts had the ability to 2 or 3 shot most well-equipped raids especially if they weren't controlled properly. They have since been nerfed. But ugh, were those guys a pain.
Again, that was just trash.
Once you got to Mag's chamber and cleared out his 5 Orc handlers (which you had to do in about 2 minutes as they get progressively stronger with each death), you'd go up against this badass himself. Most raid groups needed 2 cube click teams along with backups in case Kodo dung hit the fan.
Why it sucked
Pre-nerf, two click teams were used. Post nerf, only one was needed since the Mind Exhaustion debuff was dropped to 30 seconds instead of 90.
Mag himself hit like a freight train on the tank. Not too many tanks could sustain those kinds of blows for very long (Karazhan gear, remember). I remember having 2-3 dedicated healers on the main tank at all times. Even a simple Quake that threw off a healer or 2 was enough to kill a tank and wipe the raid.
Of course, as time went on and Mag continually received nerfs and as better gear was obtained, he became much more manageable. Blizzard themselves admitted that Mag was designed to be an entry level 25 player raid and they made him too hard. He was only accessible to the best guilds world wide.
Number 4: Loatheb
We're traveling back in time when this iteration of Naxxramas was level 60. Granted, the current version of Loatheb is in itself a pain. But this giant fungal monster was much worse. He was the last boss in the Plague wing.
I never managed to challenge Loatheb at 60 or even at 70. I'm just going by what I've heard and what I've read. The encounter was easy (conceptually). Pile in an insanely high amount of DPS on Loatheb and pray that you had enough.
Why it sucked
Healers were only allowed to cast one healing spell per minute. Any cleanses, dispels or poison removal counted as a healing spell.
That alone rendered this a sure fire gutwrenching encounter for healers. It's difficult to watch the rest of your raid take damage when you have to grit your teeth and tell yourself "No! It's not time yet!". Loatheb was an extremely precision based boss. In fact, when you get the chance, check the WoWWiki page for the original strategy and the list of consumables required. Guilds had to form for Greater Shadow Protection Potions, Bandages, and Whipper Root Tubers. I remember wandering around Felwood during the middle of the day on my Priest stocking up on Tubers.
I've never been more happier when Blizzard dropped the amount of consumables required to raid.
Did I mention one healing spell per minute?
Number 3: Eredar Twins
Don't let their deceptively attractive looks... deceive you. While these Eredar twins may have looked harmless, they've given many Sunwell raiding guilds nothing but the greatest of headaches. Lady Sacrolash and Grand Warlock Alythess were the fourth boss encounters your raid would face entering Sunwell.
This was one of the first few fights in the game where the kill order of a boss could affect the loot table. Taking down Alythess before Sacrolash was the easier of the two. If your raid group managed to take down Sacrolash first and Alythess second, you could acquire more loot that way.
I did not have the (mis)fortune of experiencing this fight pre-nerf. The strategy rumors I heard involved half the raid switching over to their Resto Shaman alt. We're talking about 8-10 healers overall. This fight was extremely intensive and straining on many healers.
An alternative form of completing the encounter involved executing a strategy that revolved around... the ledge boss. I know some of our veteran readers understand what I mean. Yes, it was the dreaded ledge boss. It was a precise movement that involved stacking directly on a ledge and being tucked in as close as possible against a certain pillar. Step too close meant beginning the encounter and an automatic wipe because the player would fall in. Not close enough meant a fairly quick death during the fight.
Why it sucked
Did I mention that it practically needed 8 to 10 healers?
The Twins applied debuffs on players that could only be removed by taking damage from the opposite twin. For example, Sacrolash applied a Dark Touched effect that reduced healing by 5% (which stacks 20 times). The only way to remove it was to get hit with fire damage from the other Eredar twin. Similarly, Flame Touched was another stacking debuff that caused players to take 300 fire damage every 2 seconds which could stack 20 times. To be fair, it had a cap of 6000 damage every 2 seconds. Again, the only way to remove it was to take Shadow damage from the other twin.
Those two debuffs alone were a big pain. If your guild could successfully beat the ledge boss, then it wouldn't be as big of an issue.
Did you get Shadow Nova? Run out of the ledge quick before you explode the raid! And let's not forget about the various Shadow Images that spawn nor the various Flame Sear debuffs that were placed on the raid.
Overall, the big reason why Eredar Twins sucked for healers is the insane amount of healing per second output that had to be brought in just to counter the effects. Not only that, if your raid was lucky enough to finish off the Twins, you'd meet M'uru next who would compound your healing corps even further...