Just Dance 2020

This requires you to score "Megastar" or better on every base song in the game. It's worth noting (as hinted in the description though this wasn't clear to me) that alternate or extreme versions of songs do not count and are not required.

Megastar is when you score 12,000 or more points on the song.

I also believe playing in World Dance Floor doesn't count, or at least I had mixed results there.

There's 41 songs (listed below sorted by length):

1:28 Baby Shark – Pinkfong - (Duet)
2:23 Infernal Gallop (Can-Can) – The Just Dance Orchestra - (Duet)
2:43 Con Altura – ROSALÍA & J Balvin Ft. El Guincho - (Solo)
2:43 Old Town Road (Remix) – Lil Nas X Ft. Billy Ray Cyrus - (Solo)
2:44 Skibidi – Little Big - (Duet)
2:45 Soy Yo – Bomba Estéreo - (Solo)
2:47 Everybody (Backstreet’s Back) – Millennium Alert - (Quartet)
2:47 Só Depois Do Carnaval – Lexa - (Solo)
2:50 Sushi – Merk & Kremont - (Solo)
2:55 Bassa Sababa – Netta - (Solo)
2:58 7 rings – Ariana Grande - (Trio)
2:59 Stop Movin’ – Royal Republic - (Trio)
3:04 365 – Zedd & Katy Perry - (Solo)
3:04 MA ITŪ – Stella Mwangi - (Solo)
3:04 Ugly Beauty (怪美的) – Jolin Tsai (蔡依林) - (Trio)
3:04 Vodovorot – XS Project - (Solo)
3:07 Bad Boy – Riton & Kah-Lo - (Duet)
3:08 Fit But You Know It – The Streets - (Solo)
3:12 High Hopes – Panic! At The Disco - (Quartet)
3:14 Kill This Love – BLACKPINK - (Quartet)
3:15 Con Calma – Daddy Yankee Ft. Snow - (Duet)
3:15 Keep In Touch – JD McCrary - (Solo)
3:16 bad guy – Billie Eilish - (Solo)
3:16 Rain Over Me – Pitbull Ft. Marc Anthony - (Solo)
3:16 Talk – Khalid - (Solo)
3:16 Into the Unknown – Disney’s Frozen 2 - (Solo)
3:19 Always Look on The Bright Side of Life – The Frankie Bostello Orchestra - (Quartet)
3:19 Policeman – Eva Simons Ft. Konshens - (Trio)
3:22 God Is A Woman – Ariana Grande - (Solo)
3:27 Fancy Footwork – Chromeo - (Solo)
3:32 Le Bal Masqué – Dr. Creole - (Quartet)
3:33 Just An Illusion – Equinox Stars - (Duet)
3:34 I Am the Best (내가 제일 잘 나) – 2NE1 - (Trio)
3:35 My New Swag (我的新衣) – VAVA Ft. Ty. & Nina Wang - (Solo)
3:37 Bangarang – Skrillex Ft. Sirah - (Solo)
3:37 FANCY – TWICE (트와이스) - (Trio)
3:37 Taki Taki – DJ Snake Ft. Selena Gomez, Ozuna, Cardi B - (Solo)
3:38 Tel Aviv – Omer Adam Ft. Arisa - (Trio)
3:46 I Don’t Care – Ed Sheeran & Justin Bieber - (Solo)
4:08 The Time (Dirty Bit) – Black Eyed Peas - (Quartet)
4:11 I Like It – Cardi B, Bad Bunny & J Balvin - (Trio)

Earn Megastar or better on each of these and the achievement is yours. You can see if you've Megastar'd a song if when you look at it in the menu there's 5 pink stars on it, as well as the third icon on the rewards for the song is not greyed out. As an additional reward, when you megastar a song, you unlock a gold avatar for that song.

You can also look at the achievement tracker to see how close you are (each song you Megastar increments that tracker by ~2-3%)

In terms of advice on how to score more: honestly just keep practicing. For me the last song was 365 (Katy Perry) which is reportedly an "Easy" difficulty song, but still took me *many* attempts. In the end it was just correcting one or two movements that made the difference between barely superstar'ing the song and barely megastar'ing the song. If you find yourself playing a song over and over and not seeming like you're improving, try changing your movements fairly radically to see if you get more "Super" and "Perfect" moves. Sometimes following the dancer can be misleading.

As well, be sure to nail those gold note sections (the ones that go "Yeah" when you hit the move correctly) as those give more points than normal sections.

Good luck! This is hands down the hardest achievement in the game (though not the grindiest).