I got this achievement early on, when there was a quest for which you had to use a power up or learn how to destroy a boon. (I'm not really sure which of the two it was) Anyway, all of the opponents were wiped out at once, which earned me the achievement. It was in one of the two first 3 stage-campaign missions.
If you miss on of those two, you can simply progress a little further, until you have powerful units. Then redo the first campaign missions, the hostile slingers should be no match. Alternatively, hope for a boon to show up right at the start of a battle.