Just keep pressing X and R2...that will help you stick with the offender and make their shot difficult and I often use "O"  to tackle the ball if i have a good chance to steal.

To shoot better (even in free kick) I use "O" and tend to press the "L" stick forward to provide accurate height..but of course you cant be pressing O for too long ...just tap it (depends on how far the post is). I hope you understand....lemme know if you wanna know more abt the controls.