I did this achievement today with the help of the videos of ngtzombies,
1st Step: First you will need to go to the power room and activate the power, when you do this 4 switches in the spawning room will pop out. Now all of you will have to hit the switches at once to go into eclipse mode.
2nd Step: Go out of the spawning room and head towards the MPL, once at the MPL you notice there is a demolished building, one the side of the building with the mummy statue there will be a button that just appeared. Now push the button.
3rd Step: Once you have pushed the button you should notice on the ground there would have been some stone tiles appear from out of the ground, when you stand on one of these tiles you should notice a symbol will appear on it, now have one/two of your teammates to go onto the other side of the map where the bridge and the waterfall is. If you go to way to the water slide and drop off the little ledge, there will be 9 tiles here, one will be in front of the gong to, now if you go across the the bridge there will be three on the other side need to the mud room. Now the person still on the side where you pushed to button should step on one of them and see what the symbol is, then step off of it and tell the people on the other side of the map, and get them one at a time to check the stones until they find the match, then will there still standing on there tile step onto your and it should sink back into the ground, but make sure two people aren't on the tiles and the same because if you mess up you have to restart this step.
4th Step: Now if you did the step above correct, it should bring you back to the present, you will need to go back to the spawn area and push the buttons to go back into eclipse mood, once u do that get 3 of your teammates to seat at the entrance to the water slide, while you go to the base of the water. Once there you should see this rock thing in the middle of the water, if you go on one side of it there should part of a destroyed brick wall. When you get close to it, it should come up "Don't Hold X", when u see this hold X and tell your team to go down the slide, once there at the bottom tell them to stay where the land, once you hit it run back up the hill and across the bridge then threw to mud house to the water slide, once there go down it and look at the right wall and hold X all the way down. When you do this you should hit a lever on the way and complete the step.
5th Step: Next go hit the buttons to go back into eclipse mood, when you do that there will be a meteor smash into the map, now go to the water slide. On the top right side of the mountain there will be this silver orb there now. First you want to hit it with an explosion to knock it down, once you do that his it will the shrinker gun to make the orb smaller, then knife it in the direction of the water slide. Once u do this it should go down the slide into the geyser and up the geyser to land on top of a pole. This is the end of this step
6th Step: This next step will involve a napalm zombie, at the end of the round you will want a crawler and the napalm zombie, but make sure you don't get to close to the napalm zombie, if you do he will just explode and you will have to wait for another, but once you have both go hit the buttons to go back into eclipse mode, next head into the cavern and then you will start hearing them coughing, there should be a value and a switch on the ride side wall. The first thing you will wan to do is to turn the value, after you do this there will be gas coming out of some pipes. Lead the napalm zombie in to the cavern you will need him all the way through the underground area until you are in the power room. 2 of the 4 leaks are hit on the stairs near the power room. Once all 4 leaks are ignited go back to where the value is and hit the switch, this will end the step and send you back into the present.
7th Step: The next step you will want to save the napalm zombie and 4 spikemores. You will need to place the spikemores in front of the holes in the cavern near the waterfall. Once you do this run back to the spawn room and hit the buttons. Once in eclipse mode go to the base of the waterfall and you should start hearing them talk.
Now go lure the napalm into you spikemores and they should plug the holes in the cavern. Once all 4 of the holes are plugged by the spikemores, go back to where the "Don't Hold X" message is, one there hit/hold x until the water pressure picks up again. This should end this step
8th Step: For the next step when you go into eclipse mode, when you do this there will be 12 stone tiles to spawn across the map, the same ones in which you step on in the 3rd step. In order to get them to activate, "light up", you have to knife them. Which some of them take a couple more knifes than other. But they are 2 in the minecart area, 3 in the spawn room, the is one in each of the passage ways with the spikes that come out of the floor. There is two near the mud room, and some down stairs. Once all of them have been activated go to the minecart, if you look past it there will be this trap looking thing, all you need to do is blow it up and that ends this step.
9th Step: Now go back into eclipse and head into the mud room the next step is to set the dials, the code for these dials is;16,1,3,4; the pictures for each one is, 1 = one dot, 3 = 3 dots, 4 = 4 dots, and 16 - [| and one dot. Once you do this your character should say it worked.
10th Step: When we did this we did all these steps in one, but make sure you have the upgraded shrinker gun. The first thing you wanna do if find the right gongs, the ones that just make the ringing sound, to activate the poles/orbs. Now you can start. You first want to go into eclipse mode, then run and hit the right gongs to make the orbs glow. Then go to the orb next to the geyser opening, you should notice there is something hanging from there, this is your dynamite, you want who ever has the upgraded shrinker gun to shot that orb, when the laser comes back and hits that orb it drops the dynamite, you want to catch it. Then run back into the spawn room and go infront of the mud room mud room in the top right on the roof of the mud room, shot that orb with the upgraded shrinker gun, this should bounce off all orbs and shoot the giant meteor and shrink it. Once u do that quickly run and activate the way to the PaP, once u do this run up there and they will start talk, once they are done talking hold x on the wall to give them the dynamite that you got just a few mins ago, after this u probably have to reactivate the PaP but once u get up there the wall will be blown up and there will be a rock spinning on the table up there, hold x on it to pick and up and unlock your 75G and gamerpic :D
In case you don't want a written guide i will attach there video of a complete walk through. I take no credit all credit goes to NGT