These videos are from pre-debut ASTRO showcases in 2015 and they are pretty amazing. In some of them, there is a 7th dancer. His name is Son Junyong, he is a Fantagio trainee who did not end up debuting with ASTRO.
This first video above is of VIXX's Voodoo Doll and BTS's Boy In Luv.
Here is Block B's NalinA.
Good Boy by GD and Taeyang.
Bang Bang Bang by BIGBANG.
Do you see how badly the stage was shaking during their performance? I'm so glad nothing bad happened.
A full cover with vocals of SHINee's View.
This last one is the Intro plus The Chaser by INFINITE. It is Moonbin focused.
Are these not just amazing? I am so impressed by these kids. This was long before their debut last year and they killed it.