Neverwinter

You will unlock this achievement by obtaining an overall Gold rating in the Incursion variant of the Prophecy of Madness skirmish.

The Prophecy of Madness skirmish can be started using your Queue. To join, you will need a level-70 character. There is no minimum item level requirement for this mission, but don't expect to get gold with a weak toon! Once you join, you will be put into one of the 2 possible variants: Madness or Incursion. You cannot choose the variant you want to play, but you can see which variant you joined by looking at the title of the quest: if it is "Demonic Incursion", you're in the correct variant for this achievement (the other quest is called "Protect the Prophecy").

The Incursion variant consists of 3 "phases". To get gold, you will need to do good in all phases. However, you won't necessarily need to get Gold in all 3 phases. For instance, I got an overall Gold rating by getting Gold / Silver / Gold. Note that I often ended skirmishes with Silver / Silver / Gold, and that gave me an overall Silver rating.

During the first phase of the skirmish, you will need to close as many portals (called abyssal tears) as you can (press on the portals). Once you close a portal, demons will emerge from them. Once you kill these demons, other portals will spawn. The scoring in this phase comes from closing portals, but you will still need to kill mobs quickly to spawn new portals.

During the second phase, you will need to close portals again... but this time the portals are bigger and spawn stronger enemies. The bigger the portals are, the more points they give. Therefore, concentrate on the largest purple portals, and then on the medium blue ones. During this second phase, do not open the green portals unless your group is melting through mobs and you are waiting for additional portals to appear.

Finally, during the third phase, you will need to kill the Balor boss. You have a limited time to take him down, and a Gold rating will be awarded if you manage to kill him within a minute.

This is by far the hardest skirmish to Gold in the Underdark expansion. I suggest getting a group of at least 2800 to 3000 item score DPS-oriented classes. Unfortunately, Tanks and Healers are not really useful in this skirmish, but they may contribute by opening portals, killing mobs, and buffing allies.

FYI, most people that have this achievement got it during the first few months after this expansion was released. The achievement was then made harder by a patch that decreased the time available to get gold ratings. Then a second patch came out, which "may" have changed things a bit (this is my understanding anyway, but it's hard to tell the difference). I got this achievement after this second patch. Therefore, it is now possible to get this achievement, but it requires more effort and planning. Good luck!!