Here is my 100% Achievement guide and full walkthrough so you don't miss a thing.
In level 7, as we fly our ship, we get introduced to the repulse o tron.
Here, we have SAM missiles coming towards us (You can tell when it gets closer as there is a little arrow on screen indicating where it is and the arrow gets whiter and whiter as it gets closer).
As soon as it almost hits you, press the
button and it should direct the missile away.
If you don't get it first time.. you will have plenty of missiles launched towards you on this mission and through the game anyway so you should get it at some point.
Go to 1:03:50 in my part 1 video to see where and how to unlock this
Hope this helps!