carcass
The video is awesome
Already seen two times

By the way mod

could you suggest me a really good source (not thousand) to tackle definitely Combinatorics/Permutations. they are my achille's hell

Good to know you liked it.
As for PnC, I think the Veritas P and C book does justice to all PnC concepts necessary for GMAT. In addition, I suggest the P and C posts on my blog and then practice questions from everywhere. There are many other books/sources but I cannot vouch for them since I haven't personally gone through them.