The Slave Pens is the lowest level of dungeon in Zangarmarsh, which is located underwater in one centralized location: Coilfang Reservoir. It ranges from level 62 to 64 on Normal Mode, and Heroic Mode is for level 70. There are three bosses located inside the dungeon: Menu the Betrayer, Rokmar the Crackler, and Quagmirran.
To enter Heroic dungeon, you need the Reservoir key which requires your reputations at first, and later on it'll be nerfed to obtain honors, and you can discover the entrance by finding the giant pipe fixture in the middle of the big lake and swimming through its orifice until you emerge in a deep Murloc Cave filled with Crabs.
The Slave Pens is the leftmost dungeon marked with a fence just outside the portal. This is a medium difficulty level of dungeon, and I won't rank it because it really depends on your composition. In general, good Crowd Control (CC) against Humanoids will help a lot as most of the enemies are Humanoids, so Polymorph, Sap, Seduction, and Mind Control will be useful.
Mind Control
Here's the Mind Control Roundup for your Priests, and keep in mind that this is for Heroic mode. Usually, you can emcee everything on Normal mode. The dungeon starts off with two patrols of crab, and you can skip them quite easily.
In fact, no cardboard box is needed. Just wait for them to walk up, follow behind, and push them into the Slave Master. If you decide to clear them all out, though all of these Crabs are Humanoids, you can see them have the most CC effects.
I happen to have a good AoE composition in this particular run, so I go with the Kite & AoE methods. They are really easy mobs to grind since they have no ranged attacks, so I recommend you do the same if you have a heavy AoE cap with area slows. Otherwise some CC may be in order and has this armor reduction ability. Keep in mind that your Tank might be taking some pretty heavy damages and is really helpful to drop a slow or stun on these mobs, so it has more control and freedom if it does Diplomatic Measures quest.
The next enemies of known are Slave Masters who will call the nearby non-elite Broken (known as Krokul in Draenei) to aid them. As soon as you kill the Slave Master, the non-elites run away, so it's not really a tough mob to handle, but if you want to maximize reputation, you should kill the Broken at first as it will give you some reputations, and you'll eventually enter the big room.
Some patrols are wandering about the room, and you can skip them sometimes. It is faster to clear them, but you can go over these enemies if you want a reputation or you aggro by accident or something.
The most dangerous in this room are the Coilfang Champion and Coilfang Rays. They're the cause of most wipes here. Someone gets fearful into the next pack. It's very easy, and they will be in your first pack to your right, so the best option is to pull them back in this hallway that you just cleared.
Menu the Betrayer
This is the first boss you'll encounter inside The Slave Pens and will be patrolling the steps, so you need to grab the three-pack at the bottom of the ramp and pull it around the corner, and then there will be a four-pack behind it, but you can skip it and grab the boss - Menu the Betrayer. You will need enough WoW Classic TBC gold to train Lightning Bolt and a few Totems. This boss can drop Rooting Totem, Healing Totem, Stoneskin Totem, and Fire Nova Totem, and then exploded, made some big AoE damages, so the best way to handle it is to drag the boss away, let him out, and then kill Earthgrab, Stoneskin, and Healing Totems.
Rokmar the Crackler
He is the second boss. The lord of the Zangarmarsh bogstrok resides in The Slave Pens.
His abilities are showed as below.
Enrage - increases his attack speed and power
Ensnaring Moss - slows attack and casting speed
Grievous Wound - target will bleed until fully healed
Water Spit - uninterruptible hurls out frost damage
Grievous Wound is probably his most dangerous ability. Usually, your Tank will go on bleeding until it is healed fully. He has lots of HP and a Frost ability, Water Spit, so don't be afraid to use these abilities, such as 
Frost Resistance, 
Frost Resistance Aura, and 
Frost Ward.
Quagmirran
The final boss is likely the most difficult part and submerged in the water pool in the final chamber of the Slave Pens. The Tank should use a ranged attack to pull the initial aggro and draw him out of the water, fighting him on the ground.
This is a single-target, low-movement encounter with moderate party damage and heavy tank damage. It's a good idea to spread out. Ranged should try to avoid his 
Acid Spray. Poison cleanse and resistance is very helpful here.
Some content of this article is quoted from MadSeasonShow's video.
Lastly, if you want to learn more TBC guides or tips, welcome to visit MmoGah.com.