Watch the cinematic.
Once you have control of your men send them south down the main corridor. You will encounter three summoners. The one in the middle will drop a valuable artifact. This is a little bonus for players who like to explore, or who can not seem to find the right way to go.
Go back to where you started and bust through the door on the west. After fighting a few murlocs you will find the hallway splits. It is easiest to go south first, then the north. The southern passage is a dead end, but you can break up an ambush before it hits you. If you go north first you will find yourself surrounded and may lose a unit.
Soon you will find another door. This leads into the spawning pools. Fight your way around the edges of the pools and try not to lose any men. When you get to the middle you will be ambushed by loads of naga. You can set up your men in a defensive formation in the middle of the torches and send one unit to alert the naga in order to survive better.
Continue your way around the pools till you find a hole in the south wall, this will bring you into a cave of turtles. After killing a few turtles you will find yourself back into ruins, there is a dragon turtle here. Kill it as fast as you can and use jav missiles to keep it stunned. Do not let it devour your medic.
After breaking back into the ruins (you must flip a switch the dragon turtle was guarding) you will find a lone murloc in a large hallway. Be cautious as it is not alone. It is surrounded by clocked murlocs. Do not let your medic get killed. Proceed through the doors to the east, then the north. You will find yourself in an ornate hallway filled with naga. After a small skirmish you will pass a large door on the south of the hall. Go inside the door (this is where the prisoners are). After clearing the room you will find cages in the southwest corner that contain the prisoners. Proceed down the ornate hallway till you find stairs. At the top of the stairs are summoners and a royal guard. Kill the guard quickly and try to keep him stunned. Do not lose your units. After you kill the guard a cinematic will start.
The tank does not turn its turret very quickly, use this to your advantage. Place one grunt on the opposite side of the rest of your forces and let the tank attack him. Use everyone else to pound the tank. Do not let the tank get a good shot at the bulk of your forces. Spread out. And do not let him get a bead on your hero. Do not worry about losing men here, this is your last fight with them.
Congratulations, you have killed the boss.
Proceed to the southwest corner and go through the hole in the wall.
Now it is just a mad dash in your tank. Enjoy yourself.