This is unlocked by playing World Dance Floor. In Just Dance 2019 after playing awhile eventually you'd end up playing against a boss, and you had to then hope that you & the other players playing at the time danced well enough to beat that boss. The same is true in Just Dance 2020, with the caveat that there have been periods of time where bosses have not been available.
The first period (from game launch until December 12th) bosses did not show up in WDF. Sources:
(see the first winners list)
And then as of December 26th 2019 they stopped, and remained unavailable until January 16th 2020.
On January 16th, the feed on the Just Dance 2020 club had a post from UbisoftOfficial which read:
"Just Dance 2020 is having a Winter Gala, and you're invited! From January 16-Feb 2, jump into the Boss Family Tournament and scope out the awesome new playlist. Just Dance Unlimited is adding three classic songs and three exclusives, including Sucker by the Jonas Brothers. Trailer's ready, see it now!"
On that post was a link to this Youtube video:
At the 0:26 second mark, there's a slide indicating that there are 3 bosses as a part of this event.
As such, presumably you have until at least Feb 2nd to get this achievement at which time bosses may once again become unavailable in WDF.
I personally unlocked it on January 21st, 2020, I went into World Dance Floor and the first song I was dropped into was the 2nd song of a boss battle, we won, and the achievement popped right after. Here's a video clip showing the tail end of the song I played, and around the 0:54 second mark is around where the achievement popped: