At the start of the third mission, "Aftermath", you'll see a burning sun-like-thing, and then you'll be attacked by a swarm of little red fighters that leave red contrails behind them. Enter Strike Mode (A) and then get a bunch of these little suckers in your sights, don't bother counting, but there are enough of them that you should get five. Now hold RB and then release RB to fire. Keep doing this (RB/release RB)
If the achievement doesn't pop after the skirmish is done, I would reload the checkpoint and try again.