You'll earn this for completing Sastasha for the first time (it's part of the main quest, so don't worry).
You'll need to be at least level 15 and synced down to level 18 if you're any higher. The dungeon is pretty straightforward and requires no real strategy most of the time, there are a few areas that can be intimidating.
Here are the bosses and tips to beating them.
Chopper, a giant coeurl. He's pretty easy, only has a single AoE attack so he won't be that tough. Just keep hitting him and stepping out of the AoE (as always, designated by the big ass red circle.
Madison #1, pirate leader, take out his henchmen and then put the beat down on him. He's not very tough so there shouldn't be any issues.
Madison #2, again, take out his guards and then lay the smackdown on him. Once he gets low health, he'll open a nearby room with several guard dogs (to the left upon entering the area). Once he releases them, have your tank pull the aggro or your team may be hurting, make sure your healer is on the opposite side or they'll pull a lot of aggro quickly.
Denn is the final boss, a giant.... fish monster thing. He's pretty simple but has a few tricks.... one trick. At various points, you'll be notified about the water bubbling (there's one right in front of you as you enter the area, three others are nearby, in the corners of the area). When you get this notification, the members of the party must run to them and interact with them to prevent new enemies from joining the fight.