Blackwing Lair

Before entering Blackwing Lair, defeat Onyxia and collect Onyxia Scales to craft Onyxia Scale Cloaks. These cloaks are required for several fights inside the raid, so equip them on your tanks before proceeding.

Enter Blackwing Lair and work through the trash to reach Razorgore. The mobs during the Razorgore encounter have roughly 7,000 health and can be soloed without much difficulty.

Defeat Vaelastrasz the Corrupt after Razorgore. At max level he falls quickly, typically getting off only one Burning Adrenaline cast before dying. Note that Burning Adrenaline also affects Death Knight resource generation.

Push through the Suppression Rooms toward Broodlord Lashlayer. These rooms remain notably annoying to traverse, but Broodlord Lashlayer himself is a straightforward kill.

Kill Firemaw, Ebonroc, and Flamegor in sequence. At max level these three function as little more than large trash mobs and pose minimal threat.

Defeat Chromaggus, whose breath abilities are not a significant threat at max level. Continue past him toward Nefarian's throne room.

When Nefarian is landing, anyone without an Onyxia Scale Cloak should hide behind the throne. Once he lands, the fight is a straightforward tank-and-spank encounter. His class calls are not a serious threat at max level, and defeating him completes the achievement.