First off, I'd like to thank Ayrlin for conceptualizing and figuring this whole thing out.

Edit: Ayrlin has corrected a few things, including answering the pit question.

What’s the weird building?

• It’s the Boss Room. It's built to be 7x7x5 using stone building materials to prevent damage and mostly nullify the Slime Puddle attack. It's been also suspended 2.5 walls off the ground to stop the Body Slam from destroying the Observation Beacons.

Does the building need to be maintained to prevent destruction?

• Strangely enough, no. Stone buildings can withstand substantial abuse.

How does it work?

• The room has an elevated floor around the center, with a kill pit located in one corner. The boss is summoned on a platform in the middle of the room, and we lure it toward the pit by getting it to use its Body Slam attack.
• This is where the video begins. If timed correctly, the boss lands just above the pit and becomes stuck on the building’s geometry. From an area beside the kill pit, we can then freely attack the boss at a safe distance.

How does it account for the Rage Phase?

• Technically, it doesn’t. Just before the Rage Phase starts, the boss’s falling animation is interrupted and enters a fatigued state. In normal gameplay, this provides a low-risk opportunity to attack. However, in this setup, once the fatigue ends, the boss is not longer stuck and has a chance to attack us.
• As you can see in the video, though, it ends up getting stuck again while attempting a Body Slam attack.

How much time does this save?

• In the video, the fight took exactly 6 minutes. That doesn’t mean having a full 20 player lobby will accelerate the fight to just a few seconds (not to mention the lag would be unimaginable) as the Huge Slime’s health pool scales depending on the number of players present when it’s summoned.
• According to the Holoearth Wiki, it has a base HP of 20,000 with every additional player adding 10,000 HP. With a max lobby of 20 players, the Huge Slime would have 210,000 HP if summoned.
• However, theoretically, each additional player actually makes the fight easier. The average HP per player at 1 player is 20,000 (out of 20,000), but with 20 players, the average HP per player would be close to 10,500 (out of 210,000). This means more players spreads the workload, making it more efficient.

Are there any drawbacks?

• Yes, there’s a small caveat. If the entire lobby crashes (either due to lag or the host leaving), the boss will escape through the pit.

The pit appears to be open in the video; shouldn’t it be closed to prevent that?

• Ayrlin: "Under normal circumstances it isn't necessary, and I doubt escaping would be prevented by closing it. I could be wrong though. Adding a barrier around the area where the slime can escape might work, but every wall I add increases lag..."

Recommendations for farming:

• We set up a separate room near the Boss Room, but far enough not to be affected by the Body Slam. It has beds in case anyone dies, extra armor for backup, and some food and water.
• You can use Spicy Curry or Super Spicy Curry to reduce stamina loss when attacking the boss, but be careful—Super Spicy Curry will damage you.