This achievement unlocks after finishing the quest Temple of the Primordial. 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 Mount Hotenow. The enemies here are 56+.
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.
A Fire in the Mountain (This is the quest Knox gives you to start this quest line)
Missing in the Ash
Knowledge From the Ashes
Venting the Underneath
Shadowfire Cavern
Seeds of Rebirth
The Flames of War
History of Hotenow
Fireguard Fortress
The Fire Fighters
The Fire Lords
Secrets of Fire
Temple of the Primordial
Note : Quest names were taken from the Neverwinter Wiki :
http://neverwinter.gamepedia.com/List_of_quests?cookieSetup=...