You'll accumulate a fair few kills towards this achievement just by naturally playing the game, but while you're probably better off getting these kills in Galactic Assault for the easier defeats, the fact that you have to save battlepoints to become a Hero probably makes Heros vs Villains the more reliable way to actually grind this achievement.
Some people are really good with lightsaber heroes, but I found if you upgrade a blaster hero on both sides you can easily chip away at this achievement while grinding XP, credits, and elusive Choose Your Path kills
Personally I used Boba Fett with upgraded wrist rockets (extra rockets and hero damage) and Chewbacca with upgraded health and Furious Roar. Always use the "zoomed" tight spread on Chewbacca's bowcaster, it can do ridiculous damage when combined with Furious Roar.
Even if the team didn't win the match, I could always spam out 10-15 hero kills each round. If you play defensively on your own team (trying to protect your hero instead of killing the target), the game usually lasts longer and racks up more kills/XP too.