Fastest boosting method.

Form 2 groups of 5 people.
1 group are people who want to derank, the other are people who want to get boosted.

Both groups search for a map such as de_vertigo or cs_militia, chances are that both groups will get matched up with each other.

Derank group kill themselves during buy time to give boosting group easy and fast 16-0 win.
At the end of the match, all people will press F1 for a rematch.

Rinse and repeat the above. Win win situation.
Fast and easy wins for boosting group, fast deranking for deranking group.


Of course, the above method has some flaws. For example, both groups will have to be of similar rank.
But if you can overcome the problems then this is the best and fastest way AFAIK.