Rescue Operation

Simple solo guide (will take most of the battleground to complete this achievement):
1. Run into the enemy keep, popping defensives and gapclosers to get as close to the keep as possible. Stealth may be an option, I have not tested but an invis potion did not work for me.
2. Ignore spirit healer, run back to your corpse (if an alliance player removes your insignia you must get rezzed at spirit healer and try again. Keep checking that your gravestone icon is still near the keep)
3. While dead, run inside the keep and turn right down to the basement. Respawn in the basement.
4. Kill the one guard there for a 100% drop key. Free one of the 3 prisoners.
5. Wait 5 minutes for the guard to respawn, kill again and free the remaining prisoners.

Turns out there's a super easy way to cheese this achievement. All you need to do is have or make a friend who is a Paladin. Have them get the [item=112005]. Then simply duel them and have them cast it on you. After a brief delay, you'll be ported to the enemy's base and the hard part will be done.

I wouldn't be surprised if the skill were spell-reflectable, so as a Paladin you can likely get a Warrior to assist you as well.

-------- Original Post below
Alright so let's cover the details of how to get this achievement. It isn't hard... if you're totally stomp-festing the Alliance and your group wants to do it together...

First, the det's: you need to rescue the three prisoners. They are inside the basement of the main fortress in the Alliance camp. To get into their room, you enter the building from the front and there is a staircase in the back-right of the initial room. You go down that staircase and there will be a single [npc=80825] at the bottom of the stairs. He's a bit of a chump -- an easy solo for my fully honor gear Rogue with no CDs used and full HP when it finishes.

He drops a [item=116140]. It's a shared drop among everyone present (no roll for it or anything), so if you have 3 people present you can knock out the achievement all at once (although it only seems like 2 of the NPCs spawn at any given time... there's nothing to stop you from just waiting for 5m, though!)

Let's assume, though, that you aren't enjoying a complete faceroll destruction, this achievement is nearly impossible to solo! Well, unless you're a Rogue/Druid of course, in which case it's super easy to solo! Even if you aren't a Rogue/Druid, you can likely follow my guide below with a Druid friend - you rush in, die, and they can stealth in behind you and ress you! Yay! A Rogue can also help you in via Shroud of Concealment, of course.

Since I'm a Rogue Ninja, I'll cover the details of the achievement under the assumption you or your friend has stealth.

Let's begin!

First, you'll want to get [spell=169134], obtained from clicking on the [npc=84465] spawns in the Molten Quarry sub-zone area. It *technically* isn't needed if you are fast on your feet, but it helps immensely in avoiding being detected by jumping up the wall instead of walking in the front door.

You then enter the building. To do this, you can enter the front door (hug the left -- there is a guard on the right). I suggest anything you can do to improve your stealth levels or speed during this portion. If you choose to enter the front door and get pulled out of stealth, you're as good as dead.

OR you can use the [spell=169134] effect. Stealth up to the northmost wall of their fortress. If you're looking at it from the front -- it's to your left. You basically "jump" up the side and land on the 'roof' area (really, the rampart area, with doors into the central building). There are no guards on the inside of this wall, so you can enter safely and simply jump down into the room below and enter the basement with ease!

That latter method is the one you would need to take if you don't have stealth. Taking something like a [item=116925] will help you out as well, since the guards will root you on sight. This will help you get into the basement before you die! Alternatively, a Rogue can help you in via Shroud of Concealment.

I know this can be a bit confusing, so if someone needs me to make a video or something I will. I'll quickly toss up some imgur pics below so you can get the gist, though!

Here are the pics:
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EDIT: I should add, too! There's a respawning Dusty Scroll in the cell with [npc=86667]. You can simply open the door and walk in to loot it. It gives you 1x[item=116413], which will let you easily escape when you finish. The scroll's respawn is about 1 minute, so feel free to stock up before you leave (or between respawns!)

You can also get to the prison by getting hit by Jailer's Judgment from an opposite faction Paladin and not breaking the stun. That's how I got inside the prison on my warlock, and was easily able to do this achievement afterwards.