As MegaManMasterer said, simply use block (LT) shortly before the missile hits to deflect it back to the AT-MP. It will take two or three times for you to get the right timing, but it's pretty easy.
The best and quickest way to get this achievement is to choose the Deflection Challenge from the Challenge menu. The first several enemies to appear will be AT-MPs and you'll be given plenty of opportunities to deflect the incoming missiles. I went in to this challenge with ZERO prior deflections and the achievement popped in about one minute or less, before the AT-ST even showed up.
Good luck!