Ruins of Ahn'Qiraj

This guide is made so you don't have to read in detail every boss fight 1 by 1, as that takes too much time, and I also included a walkthrough. Did this last night to get our achievement done was cake with the 3.02 patch. Group consisted of all 70's:
MT= Ret Pally aoe tanking in most s4/ some s3 gear
MH= Enhance shaman but healing in T6 gear
DPS= Fury warrior (some blues and epics), T6 fire mage, T6 dest warlock, s3/s4 Mut Rogue (me), and ret pally that just hit 70 with only 6k health.
To get to Ossirian, you only have to kill TWO bosses before him, the other bosses are optional. The first boss Kurinnaxx, he is tank and spank but added a rediculous debuff that reduced chance to hit by 75%, but easy non the less.
After he is dead some Cen Circle npcs will appear after a while to aid you, although these guys help you kill the waves they aren't really needed. The room after Kurinnaxx starts the event for the 2nd boss, although it looks like you'll pull everything, just pull the first group, which starts the event with multiple waves. Kill the giant White Bug guy first, we had our warlock and warrior mass fearing the other adds. After the insane amount of waves, General Rajaxx will appear, be warned he does a knockback that hit me for 2k, and caused our tank to lose aggro right away.
After the Genral is dead, this is were you can SKIP to Ossirian the Unscarred, the room that had all the waves coming from, turn right, and stay a long the left side killing adds until you reach a giant stairway, this is Ossirian's room.
Trash in here may reflect certain school or spells so read their buff before attacking them. To kill Ossirian check: http://wotlk.wowhead.com/?npc=15339 , search him in wowhead and see other people's comments. With even our small group we got fed up and just burned him down after 25% with no crystals. Hope this quick guide helps getting your achievements.

*IMPORTANT NOTE* The head that is dropped, http://wotlk.wowhead.com/?quest=8791 was NOT needed to complete the achievement, Blizzard must have changed this since the head can only be looted by one person in the raid. Well hopefully this helps, as we did this with only 7 people.
--Good Luck

I did this as a shadow priest at level 85 solo and most other casters should be able to do it pretty easily. So here is a casters guide:

Killed Kurinnaxx then managed to sneak past most adds until I reached the room with the final boss,[npc=15339] . Clear the entire room so none of the adds mess with you during the boss fight. Pull the boss then run to the nearest crystal sticking out of the ground and right click to activate it. This puts a debuff on the boss making him weak to one type of magic (i.e. fire, shadow, frost, etc.) keep running with him behind you, activating crystals, until he becomes vulnerable to the type of magic you use most. Once he is vulnerable you should be able to dps him down pretty quick and get the achievement Ruins of Ahn'Qiraj and [item=21220] which rewards rep with Cenarion circle and Brood of Nozdorum.

Good Luck!

I would like to point out that this achievement can be soloed by a level 80 feral tank (and surely a few other classes) in Naxx10-ish gear quite easily. The parent post describes very accurately the shortest path to killing Ossirian. Rajaxx was the hardest part of the whole experience for me, as he crit for 17k at least twice and regularly hit for 8k (once every ~20-30 seconds?). I had to pop all cooldowns and nearly died within the final 5%, but no potions or other consumables were used. Ossirian was a joke to take down. Spam DPS while kiting to crystal, wait for buff to go up, use cryst