First, 400 Teensies required to reach the world
"Living dead party".
This is the last world of
Rayman Legends.
You need to finish all levels, but it's not necessary to catch the 3 teensies in a level, or reach the 300/600 lums.
You'll replay each musical level of each world of Rayman Legends + 1 level "Grannies world tour" in normal gameplay, and 8bit gameplay.However it will be necessary to you to complete the level
WITHOUT DYING
!
To complicate even more the task, the levels will be strewed with small defects, what will oblige you to
use the music of the level more than your eyes.
I recommend you to train on the standard levels to memorize the difficult passages.
Don't forget : These levels are hard and frustrating, so keep calm if you loose, try again after few minutes, and replay or watch the normal level help a lot.These videos are in the order of appearance, but
you need to complete a level to unlock the next.This is the 8 bit version (except the first), watch them to make you an idea of the nightmare

The number for the difficulty are based on my opinion, but don't reflect the real difficulty, some people will find these levels easy, others will find too hard.
- Granny world tour - normal (difficulty : 4/10)
- Castle rock - 8bit (difficulty : 4/10)
- Orchestral chaos - 8bit (difficulty : 7/10)
- Mariachi madness - 8bit ( difficulty : 5/10)
- Gloo Gloo - 8bit (difficulty : 6/10)
- Dragon slayer - 8 bit (difficulty : 9/10)
- Granny world tour - 8 bit (difficulty : 12/10 ,
YES 12 
)
Videos (
Credit to : IGNGameplay, Shadow21gaming)