I went for this near the end of the game. This allowed me to have some upgrades to increase duration of the slow ability. I also had extra stim packs so I could regen the force bar if I ran out.
I strongly recommend doing this on a normal stormtrooper, rather than anyone else. I tried on a few different types and it didn't work as even if I could hit them with their bolt, it didn't kill them.
Firstly, lock onto the enemy with R3. Then when they fire, press R1 to slow them down, which also slows down their blaster bolt. Make sure not to move so you are inline between them and their shot. Now press R2 to pull them and they should fly into their own bolt and die.
Other guides here have a video of the process if you need further help. Figured a text guide would be useful for those of you who don't want to watch a video. I highlighted the method in bold for anyone who just wanted to know the process.