In the mission "Ashes of the Empire" - There is a checkpoint right before a barrage and enough enemies to get this done (It counted stormtroopers and the guys fighting them for me)
The checkpoint is useful because they dont always group up the best. Aim for the group in the Middle at a lower elevation than you.
I tried this on the level where you steal tie fighters, as they give you a barrage then, but they dont like to spawn in groups of five on that mission.
GL!