Once you've unlocked Kylo Ren, face a hostile character. It should glow with a red aura. Hold
and the achievement should pop.
Please note: this seems to have not popped for some people. I did this in solo free play against a hostile Stormtrooper. The Stormtrooper had a red aura, and then I held
and it popped right away. I don't know what I did differently from others. I don't like giving bad advice, so if anyone has an idea, I'll be sure to update the solution.