Prison Break: The Conspiracy

You'll probably end up getting this while going for the underground fights achievement, it's worth noting that you're allowed to be knocked down and have to rapidly press "Y" to get back up.

The easiest method to get this is in the underground fights, keep fighting Soda Pop, he is very slow and also the weakest of the enemies you can choose from.

This video shows you the general idea of what you should be doing.

*^{Credit to lordGTAwarrior for the video}

The description is confusing and not what it seems.
Getting knocked down does not mess up this achievement.

Knocked out means getting knocked down 3 times (Shark difficulty, nor sure about the others) or failing the button mashing sequence when knocked down.

I got this achievement after my 5th fight in the story, but you could also do this underground by beating Soda-Pop 5 times in a row, since Soda-Pop is very very slow and you should not have any problems winning the fight by hammering the A-button.