At the top of the very long zip wire on Kashyyyk are 3 rocket troopers. This is the best place to grind it out if it doesn't come naturally. There is a meditation point at the lower end of the zip line to respawn them.
To get the kills, slow time
as a rocket is launched, then use force pull
to grab the rocket and then force push
to send it back. You should kill all 3 troops with 1 rocket.
Rinse and repeat.
Thanks to Pink Freud PhD for a few other tips for this achievement. I'll paste them below to save opening the comments section (especially for mobile users)
1) Kills made by Force Pulling an enemy into the explosion don't count.
2) Explosions that kill enemies but were fired/thrown by enemies don't count.
3) Slow Time is not necessary but makes it easier.
Also as CERBERUS 1880 has pointed out, you dont technically need to Force Pull the rocket (or grenades) first, but it will help, especially if you need to readjust your aim.