Just /Dance

this is obtained by having one person /dance with the discos on the sides of the boss room during the intermission phase.

Step by step guide for all the achievements in Glory of the Liberation of Undermine Raider: https://youtu.be/LotBGmWtn-o

There’s two disco balls, one on each side of the arena. 2 brave players will need to go up to the disco balls (one person per disco ball) and /dance with them when the boss is casting Hype Hustle during the intermission phase. Ideally it’s players with immunity who do this, or players that can run fast back to the lit amplifiers, or just take one for the team :) When the 2 players /dance with the disco balls they will light up with a colourful beam and the achievement tracker will tell you that the achievement has been met. Then just survive and defeat the boss.

There's 2 disco balls (1 either side of the room), that you have 2 people go /dance with (targetting the ball!) during the intermission.
It seems you have to be pretty close to them (like... right underneath them) for it to work, and they gain a flashing disco light 'world marker' beam over them when they're successfully activated.

In both our cases, the dancers died shortly after the /dance, due to the waves in the intermissions (the first guy derped somehow, and someone else had to take his place... and I wasn't paying attention to move to the safe spot, after using turtle, due to the other one not triggering and me trying to work out why on the fly >.< ).
We knew this didn't matter though, so long as we got the dance off, it didn't matter that we died (we were expendable in that regard :P ).
It's still better to try choose people who can probably survive it, like using immunities (pally bubble/hunter turtle etc) if you can :)