Knowing what to do, this is, without any doubt, the easiest achievement of the game! All you need is a second controler...
So, here are the steps for a failsafe method:
1.Create a player as tall as possible.
2.Start a game with the team you put the tall player against any other team (don't worry about any sliders, that's really not necessary). Let your opponent set in the second controler.
3.Let the other team have the ball.
4.Select the tall player and leave him waiting in the middle of the court.
5.Using the second controler, bring the player holding the ball next to the tall guy. This step matter the most! You need to align the two players perfectly. Use the circles on their feet to guide you. Go ahead with the next step only when you're certain that the players are well aligned.
6.Hold both controlers and press and hold X in the second one at the same time you press Y in the first one. This way the timing to your guy block the throw will be perfect!
7.Repeat steps 5 and 6 at will...
It doesn't matter if the players are out of the 3 points mark. The block still counts. Doing it in the middle of the court avoid others automated players to mess things up.
I was able to perform 8 blocks with the same player in less than 2 minutes. And that's because I screwed up a couple of times with the alignement of the players.
If you're confident and that's the last achievement you need for the game, set the quarter to 1 or 2 minutes lenght. It will be more than enough...