To do this quest, you need to complete AF1 (Blessed Hammer) and talk with the prince in San D'oria. The final battle with the NM is tough and you will need at least 12 people.
You said to do this quest (AF3 Healer's Briault) you'll need at least 12 people. But what are the level range for these 12 people? Level 50's? Level 55's? Level 60's? Please be specific. Thks! =]