This achievement unlocks after finishing the quest Scoundrel's Retreat. The quest line that leads up to this starts with General Knox in Protector's Enclave, as all story quests do, and takes you to Pirates' Skyhold. The enemies here are 42+.
I believe these are the quests you need to do, it's hard to tell since the game gives no indication of what are story quests and which are side quests, that I can see.
Company Yargo (This is the quest Knox gives you to start this quest line)
Hostile Takeover
The Scaled Menace
A Skyship's Remains
Reclamation
Ahead to Skull Fortress
Eviction
Breaking the Blackscales
Beneath Skull Fortress[2]
The Dead City
Blackdagger Buccaneers
Saving the Living
The Inside Job
Opposition Research
Shattered Ships of the Sky
Scoundrel's Retreat
Note - The quest names come from :
http://neverwinter.gamepedia.com/List_of_quests?cookieSetup=...IGutlessIWonder says : Lili Huntinghawk, that's the girl who has no indicator above her head that you need to talk to near Salvage Camp just before walking into Sunken Town. Also, it's worth noting that any quest-givers in the game have gold lettering in their names, just something to keep an eye out for if others are having trouble where to find the next quest.
Talking to Lili seems to have fixed the problem for a lot of people, so give it a try if you are stuck.