This and other guides can also be found on the Discord that was thrown together[discord.gg]. Below are the current steps.
See Next Post Specifically for Keycard Locations

Filling the Soulbox near the elevator (~30ish souls)

Requires all players to be on the elevator (may end current round)

Either Spend 3000 points on the Giant door or crawl through the far vents on the upper floor.

Take the elevator to B1. Press F on the ladder to enter the vents. Fall down inside the cells and pickup the green Level 1 Keycard to continue.

Take elevator to B2. Open level 1 Keycard door and pickup C4 (4 charges). Press F to blow up wall marked with a red X in the back corner. Blow up another wall with a red X at the end of the hallway by pressing F.

Recommend a good PaP'd gun and Perks
Survive for approximately 3 minutes trapped in a room with infinite zombies and Brutus Spawns. All players are teleported in. Afterwards Pick up Keycard Level 2 on one of the pillars to continue.

The Rear Hangar Door on the 1F is now open, once a player enters everyone is teleported into the lockdown. Boss is significantly harder per each player. Faster shooting guns work best. Destroying the hull prevents tank from moving, destroying the turret slows down its aim and reload speed. After Defeating Tank the blockage will disappear. Continue onward and Pick Up Keycard level 3.

If the chosen difficulty is Easy, Normal or Revengence, all players receive Perk-a-Holic. Otherwise 5,000 points are awarded.

Enter The Nuclear Weapon storage facility and call the elevator.
Take the NWSF Elevator to B1 and pickup the [quest item] Nikita Rocket behind a level 3 door.

Press F to use the rocket on B2 to deactivate the Electrified Floor. Afterwards, proceed to the Lab and defeat all of the Brutus that spawn in this lockdown. After they have been defeated, approach the locker in the corner of the lab and Press "F".

Take the elevator back to B1, hang a right and enter the now accessible Commander's Room. Upon entering, all Players will be teleported into the lockdown. Depending on Boss health, boss will teleport when a player looks at it a number of times before stopping for ~4 seconds. Dodge the flying items that move across the room. The Boss is always damageable but you'll miss out on the short damage phase if you are reloading.

Regardless of Difficulty, all players receive Perk-a-Holic

If you have gotten keycards 4-6 proceed through the secret door and travel through the cave, underground passage and climb the Communications Tower.

When a player reaches the Radar dish all players will be teleported to the roof. The HIND will attack the players and has 3 Phases.
Phase 1 (First 50% Health) - Missile Center Platform, Machine gun sweep, high-salvo attack, Side Machine gun sweep, Surprise rocket.
Phase 2 (25% Health) - Homing Rockets, Machine gun sweep, high-salvo attack, Side Machine gun sweep, Surprise rocket.
Phase 3 (Final 25% Health) - Constant 360 rocket sweep.
After Defeating HIND - players are awarded with Perk-a-holic and Keycard 7.

Descend Communications Tower B. Traverse across the snowfield and enter the northeast building. Descend further past the blast furnace, cargo elevator shafts A and B, through the Warehouse and Spillway and enter the Underground Base.

Find the Normal PAL key in the Underground Base. It is the yellow summoning key in the dirty water. Take it to the control room and enter it into the first terminal.

Stay inside the Warehouse while the key freezes. The blue keeper pedestal will rise out of the floor to indicate the progress. Once notified of completion, go back to the control room and enter it into the middle terminal.

Stay inside the lava floor of the Blast Furnace.The red keeper pedestal will rise out of the floor to indicate the progress. The more players in the lava the quicker it rises. Once notified of completion, go back to the control room and enter it into the last terminal.

Interact with the panel beside Metal Gear Rex. A cutscene will play and all players will be teleported into Rex's Lair. The Boss has 2 Phases.
Phase 1 (First 50% Health) - Destroy the Radar dish
Phase 2 (Last 50%) - Defeat the Pilot

After defeating Rex, the side room will open to a garage. Interact with the passenger side of the jeep to end the game.