Just Dance 2020

This achievement is for scoring Superstar while in co-op mode on a song. You can select co-op mode when you pick a song and are choosing which dancer you want to be, there's a "Press X to turn on co-op mode".

The big difference in In co-op mode instead of separate scores for each player, there's a single score shared for all.

In single player Superstar comes after 5-stars (ie it's essentially the 6th star), and happens when you hit 11,000 points on a song (I suppose then in co-op with 2 people it's 22,000? Not sure). You'll know you hit Superstar as it'll flash "Superstar" on the screen where your dancer is, and instead of 5 gold stars you'll have 5 blue stars.

Note that for co-op mode to be available:

A) the song has to have at least two dancers (duet, trio, etc)
B) there has to be two physical players

As such you need a buddy to do this. Well, sorta, you can swing multiple players if you have multiple phones. If you do this, then hold both phones and play a solo song (since both dancers would have the exact same part -- credit to CandidFever3 for the tip).

As for good songs to do this on, that's largely personal preference. I did this with my 8-year old daughter on 7 Rings by Ariana Grande, but just pick a song you & your friend can do well on and give 'er. :)