Note: The Egg can be completed with any player count!

Step 1 - Build all the bows.
A: To unlock the Wrath of the Ancients (Basic bow), you must first feed the 3 dragon heads. One is located in the courtyard above Double Tap, one located in the Courtyard before the Comms room, and one in the Undercroft above the pyramid. It takes 8 zombies (1 to activate and 7 to feed) to complete each head. Once finished, the bow will be located behind the knight coffin. Note that you will need to grab a fresh bow for each upgrade!
B: The Lightning Bow can be started by going to the weathervane on the right side of the death machine area and shooting it with the bow. This will cause the wall to explode, releasing the upgrade piece.
Next, there are 3 pyres around the map which must be shot to ignite. The first is to the right of the Clock tower computer, just a little ways outside the map. The next one can be seen if you go to the death ray and look down and to the left of the gondola, much further out then the first pyre. The last can be found by teleporting to Launch Pad and looking to the right of the Wundersphere, a little ways up. Once all 3 are lit you can proceed.
Head down to the Undercroft and wait for the anti-gravity to kick in. once it does, you will notice 5 flashing tiles on the wall that have a symbol similar to wind on them. You must wallrun along all 5 in one go.
Now there will be 3 urns next to their respective pyre that are sparking. You must kill zombies next to these to collect their souls. These don't take a lot, and you will know you're done when a small ball of electricity is seen above the urn. When the urn is charged, draw back your bow (R1) to charge your arrow with electricity. Walk over to where you shot the pyre for that location, and re-ignite them. They will now have a fire and lightning vortex flowing from them.
Head back to the weathervane and hold square on the blue mist in front of it to charge your arrow. Pick it back up once it's on the floor again.
Head down tot the Undercroft once more and place the arrow on it's respective alter to begin killing zombies once more for souls. Once filled, hold square again to transform your bow into the lightning bow.
C: The Wolf Bow can be started by activating 4 pictures in a certain order. This will cause it's arrow location in Undercroft to burst open. The 4 locations are: Bottom floor of the clock tower, Room between Mule Kick and Courtyard, Upper left floor of Comm Room, and the last is just before the Knight Hall (same area as Sams room.) The pictures locations will be random every game, but they must be activated in the same order: Knight with wolves, knight on chariot, Eldritch Horror, Sad knight (or whatever this picture is supposed to be.)
Once you've retrieved the upgrade piece, teleport down to Launch Pad and go to where the VMP(?) is. High above in the mountain will be a flag. Shoot this flag to knock down a wolf skull that can be retrieved with square.
Once the skull is in your possession, return to Undercroft and place it on the wolf skeleton to the right side of the Wolf alter (facing the build table.) This will spawn a Wolfs ghost which you must follow to collect zombie souls. He will go to 3 locations: Courtyard, Front gate, and Undercroft. Once enough souls are collected, he will dig up a pile of bones for you to collect with square.
Now back in Undercroft, the wolf will sit in front of a wall. Wait for anti-gravity and wall run along the left or right wall of the one will the hole he is looking at. This will reveal a wolf symbol you must shoot with your bow. Once shot, land on the floor for the platform to build itself and use the anti-gravity once more to climb on top. Inside will be a skeleton, interact with it by holding square, and again when it reaches out to hand you the arrow.
Just like the Lightning bow, place the arrow on its respective alter and collect souls to charge it up. Once finished, hold square to transform your bow.
D: The Fire upgrade can be found by climbing the stairs to the top of the clock tower and shooting the wall with your bow. Once collected, head down to Launch Pad and wait for the rocket to go through a test run. Once started, be prepared to run out as you are on a timer. When the doors open, there will be a glowing rock to the right, above the VMP(?) that must be shot with the bow (be sure to aim a little above as arrows drop.) Once shit, it will careen across the map.
Next, you will be spending a lot of points on the Wunderspheres. There are 3 circles which must be ignited with your bow while flinging through the air. One directly between the death ray and the Wundersphere, one next to the Clock tower terminal, and one by the car at Double Tap.
Once all are active, you must stand inside these circles and collect souls. You'll know you're done when a beam of fire shoots out of each circle.
Head back to where you retrieved the upgrade piece and hold square to reveal a symbol. This symbol correlates to a fireplace in the map. The 3 fire places are: Comm Room, Eddies room, and the room above the Front gate dragon (Wunderfizz location.) Find the fireplace that matches your symbol and keep its location in mind. Next, find the circle that correlates to your fire place (I believe it will be the only one lit) and stand inside it. When inside your screen will have an orange hue around it, indicating you're ready to launch a lava shot. Know when you shoot, a small lava rock will appear where the arrow landed. The objective is to move this rock next to your fireplace so you can ignite it. Keep in mind you only get 4 shots.
Once done, head to the death ray to see a giant fire ball. Stand under this and collect souls. Once completed, you can interact with the ball to detonate it in the sky, dropping your completed arrow.
Head down to Undercroft to place the arrow on its alter and begin charging it with zombie souls. Once done, hold square to transform the bow into the Fire Bow.
E: The Soul upgrade can be found on the ceiling of the front gate, just after Double Tap. Shoot it with the bow to open it.
Head to the circular room between courtyard and Mule Kick to see a purple tile on the floor. You must melee a zombie above it to break it open. Once done, hold square to release an urn. Now, you must locate 6 skulls around the map that you must interact with. They can be found at: Comm Room (above in a window frame), to the left of Sam's room, before Double Tap on a ledge leading into its room, past the RK5 over a small wall, in the sink of Undercroft, and at Launch Pad in the back of a truck on the left (facing the rocket from teleporter.)
Head back to the circular room and make 6 crawlers. This will trigger an audio that you need to listen to. The Keeper will speak 3 words that you need to remember (if you didn't hear him, shoot the urn to have him repeat.) Now, you must kill a ton of zombies until they drop 6 symbols. This can take a while as it seems to be completely RNG.
Once all symbols can be seen under the skulls, head to the Knight Hall. Here, you must hold square on 3 knights (they will have symbols in front of them, you must activate the 3 with the words the Keeper told you, and in order.) When you do this, another symbol will pop up for you to remember.
Go back to the circular room and shoot the symbols the knights revealed in order. This will grant you the arrow.
With arrow acquired, head down to Undercroft and place it on its alter to begin collecting zombie souls. once charged, hold square to transform your bow into the Soul Bow.

Step 2 - Build the DG-4.
Piece 1 can be found by heading down to Launch Pad. Wait near the button to the left until a test burn begins, causing the button to activate. Hold square on it and run inside to the teleporter. Once here, there will be another console with 3 buttons slowly turning green. When all 3 are green, hold square to activate the console, spawning the piece inside the teleporter.
Piece 2 can be found by activating the death ray for 1,000 points. Once active, you will need to look in the air for this piece. It can spawn in 3 locations, and the location it spawns in indicates which Wundersphere you must take to grab it.
Piece 3 is dropped by killing the Panzer Soldat, who usually spawns around round 12.

Step 3 - The Wisp (Step 3 and 4 must be done during 1 round - need confirmation)
Grab an unupgraded bow (place your upgraded bow on its alter) and shoot the ball points of the teleporter prongs to turn them a gold/red hue. This will trigger the wisp to spawn. During this step a random item (there are 8 predetermined items) will glow blue. The locations are as follows: Radio in the corner opposite Speed Cola, Clock above Comms Room, Clock in the hall leading from the clock tower to the Comm Room, Phone between power switch and Stamin-Up, Globe by Sam's room, Box in the room above Double Tap, Car tire by Double Tap, Phone at Spawn.
One will randomly glow blue. Once found, shoot it with your bow to move it to the next area. You must shoot 4 in a certain amount of time. Once all 4 are shot, the teleporter will turn purple. Take it back into the past and you will need 3 things. The Soul Can, the tesla coil, and the safe code. Be sure to get all 3.

Step 4 - Charging the Tesla Coils (Step 4 and 5 must be done during 1 round - need confirmation)
Once back, go to the death ray and place the fuse in it. Set the dial to Protect. (There is a bug where the terminal wont turn on to input the first code. If this happens, activate the death ray for 1,000 points and they should activate.) Once it's done, go to the computer by the clock tower and enter the code from the safe by pressing square at the corresponding images. This will open the safe.
Inside the safe will be a keycard and two coils. Pick them up and go back to the death ray. Once there, look for the giant tesla coils. They will both need a coil. Press square to insert it. Set the death ray to destroy.
Now, an infinite dog round will start. You must go to the computer by the clock tower, and the one at the Launch Pad to play Simon Says. You MUST complete both perfectly. if you mess up, you have to start over. If done correctly, the green button on the back of the death ray will be lit. Activate it and this will cause the rocket to crash into the courtyard below you. Head down, and pick up the Golden Rod from the rocket.

Step 5 - The Keeper
Now you have to repeat the Wisp step. Once all 4 are hit, head back in time once more. Near the front of the room will be a black case that will open by pressing square on the terminal to the right of the room with a small spherical object in its panel. Inside is a tablet. Pick it up.
Once collected and returned, go to the Knight (next to the bow) and insert the Golden Rod. This will spawn a Keeper who will move around the map for souls in a fashion similar to the Wolf.
Before starting the area by the car, be sure to place the tablet. The other 3 locations will not require this.
When he stops, there will be a white circle on the floor. The more players inside the circle, the bigger it gets. When standing inside, there will be a color at the bottom of your screen that corresponds to what bow must be used to collect the souls. Keep using that bow to kill until filled and repeat for the other 3. Once completed, he will return to the pyramid at Undercroft.

Step 6 - The Fight
Once at the pyramid, place the soul canister on the pyramid and get set up for a boss fight. When you're ready, all players must place their Ragnarok (L1) on the gravity pads to trigger the fight.
In the center of the room will be a giant Keeper. He has a myriad of attacks plus enemies (skeletons and Panzers) to avoid. What you must do is keep an eye out for a ball of electricity in the center of the room. once there, place your Ragnarok (L1) to stop it from activating. If done properly, his chest will glow yellow. Lay into it with your bows. If you fail, hide behind a pillar to not be insta-killed by the discharge that will follow.
Repeat until he is defeated. In front of the pyramid will be a Summoning Key for you to pick up. Pick it up and take it to the bell tower computer. Once placed, you did it. Victory. Huzzah!

I hope this guide helps you and clears up any confusion about certain steps.

Update 1: The clock tower is NOT a required step.

Disclaimer: I do not own nor did i write this tut....
Props go to- PocketSword