You should unlock this in your first fight in the game, as it is really easy, even on hard.
You will still be in the tutorial section of the game and will be prompted to press
to perform a normal attack. There are 3 enemies initially. Just mash
whilst pushing the
in the direction of the desired enemy.
If by some ludicrous reason you haven't managed to get a 10 hit combo, there will be many opportunities throughout the game to get this.