BIG BANG THEORY
This is thanks to Erspence over at xbox360achievemets.org
Also Thanks to mcsportzhawk on YouTube, show him some love.
I would only go for this achievement if you have 4 people, however, you can do this with just 2 or 3 people as well. You will also get the Cryogenic Slumber Party Achievement after saving Samantha (about Half way through the easter egg)
*STEPS CAN BE DONE OUT OF ORDER*
When the power has been activated, you'll need to play a round of "Samantha Says" on the four computers in the outside area near the beginning.
Next, work your way to the labs. Here search for the "Hacker" nearby on one of the counters. Using the hacker, hack either one of the four red buttons along the wall. This will activate four switches nearby as well. You can locate them by searching for their green light that should be on by now.
Now you need to hack all four of these switches. Once this has been accomplished, the four buttons you found on the wall earlier must be pressed in less than 2-3 seconds. When done properly, they will stay lit. This will open up the round hole in the pyramid front graving. Of course you'll be needing this for another step.
When tunnel 6 has been breached, have someone hack the proper terminal from the Receiving Area so that the Digger leaves.
Go back safely to tunnel 6 and see that a Black Egg has appeared near the M16. Knife it and you'll see it moving around the room. Follow it since it will get stuck in various places. Knife it again and open doors for it until it flies up onto a dish on top of the spatial base.
Have someone use the Wave Gun on the tip of the Dish. This will cause the Egg to move again. Follow it through tunnel 11. It will go hide into some metal railing at some point. You can either shoot it from the wall next to Stamin-Up of throw a grenade at it. Keep moving it until it reaches the pyramid in the power room. It will fit in the hole you just opened.
A tubular container will rise up from the right side of the pyramid. Next step consists in filling up this container with Zombie Souls. Kill around 25 zombies near the container. You'll see the souls flying up to the ceiling structure which will fill up the tank.
Once filled up entirely, you'll hear a sound. Activate the switch right next to the container; this will open the pyramid, revealing Samantha.
At this point you will be awarded a Death Machine for 90 seconds. As well as the achievement "Cryogenic Slumber Party" unlocks.
This is only the first part of the Easter Egg. There is still another achievement to unlock! Starting from here is the part where you absolutely need Richtofen's Vrill Generator to proceed!
Head back to the "No Man's Land" (Area 51 where you began at). To your right stands a beam structure with some plates stores on it. Throw a grenade & the plates will fall off.
Have someone throw a Gersch Device behind the crates, right where the plates have fallen. If done properly, the plates should now be on the Teleporter Pad.
Go back onto the Moon. The plates should now be right in front of the Quick Revive. Have someone use a QED on them, after doing this they should now be right next to a computer in a corner of the same room.
Look in the labs for a lead pipe. It is often hidden in one of the counters where the hacker spawns. Grab it and plug it into the machine back in the Receiving Area. Have Richtofen put the Vrill Generator in between.
Have Richtofen repeatedly mash the X button on the computer keyboard for a few minutes. Wait until the voice over ends, the computer screen will STAY green.
The Vrill Generator will now be powered up and shiny. Grab it and head back to the pyramid. Get near the engraving and put the Vrill Generator in it. There will be four tubular containers rising up from the corners of the pyramid base.
Fill up the four tanks by killing around 25 zombies near each container. The same sound emits when a container is full.
Activate the switch next to the pyramid again and the containers will be drained of their content. Have Richtofen grab the Vrill from the hole in the engraving upfront. Player Richtofen now earned permanent perks!
Head back to the four "Samantha Says" consoles where you need to play three increasingly more difficult rounds.
Time to head back to the power room. Have someone use a QED on the engraving to move the black egg again. (Tip: Go prone and get as close as possible to the engraving, then aim low at the golden ring then throw the QED). If some how you do mess up with the QED and the egg doesn't move, a wave of zombies will charge at you so stay on alert.
The Egg will fly back to the outside area, right next to the "Samantha Says" consoles. Use a Gersch Device on the egg.
You'll see the Egg teleport onto three rockets outside of the playable area where they'll launch and head towards the Earth. The final achievement "Big Bang Theory" will unlock and all players will earn permanent perks. The Moon is now yours!