Heroic: Hellfire Ramparts

This seems to be the only outland heroic dungeon achieve that requires all bosses in the instances to be killed compared to other achieves where u only need to kill the last boss.

It's the same entrance-- you turn it from regular to heroic mode and back by changing dungeon modes. Right-click on your portrait in the upper right-hand corner, and you should get the option to convert from 5-man regular to 5-man heroic mode.

First you don't have to kill all bosses for the achievement in theory. Nazan and Vazruden are the same encounter (You fight first the orc Nazan and later the dragon Nazan will join the fight). You don't have to kill the Gatekeeper for this achievement. However, due the nature of the instance, there is no real "end-boss" both Nazan/Vazruden and Omor the Unscarred are at the end of the instance. One is to the left and one to the right. That's probably why you have to kill both for the achievement :)