One major point that most of the solutions here seem to miss is the fact that you NEED to do this in survival singleplayer! I played more than 5 matches in survival with a friend of mine after the DLC came out and i did certainly more than 200+ kills and still no achievement.
As a sidenote, i recommend to use either map where you can hide somewhere without too much enemy presence and play it on normal (no need to make things harder on you). I recommend Hoth Base Alpha (the first Hoth map of the Original Game). Just camp in the hangar where you could stay for the easy general akhbar and 25 minutes completion.
I recommend some kind of healing skill, Ion Torpedo or Grenade and Ion charged shots or personal shield. It is easy if you can evade the shadow troopers, especially on normal.