To earn the Back at You trophy, you just need to deflect blaster bolts back at enemy troopers. You will likely earn this through natural progression as bolt deflection is a useful crowd-clearer, plus some troopers take the high ground and can really only be defeated through deflected bolts or Force pulls.
Deflecting enemy bolts right back at the opponent works in the same way as parrying – rather than keep L1 pressed for a continuous block, you need to tap L1 just as the bolt is arriving to parry it straight back to the trooper who fired it.
If you're looking to grind this out, whack the difficulty down for one-hit kills.