The Spartans first appear in Chapter 4, Level 1. There are more than 10 to kill in this level to unlock this achievement.
By now, you should already unlock the Akimbo (dual-wield) guns, energy shield and the slow time special. This combination would be good enough to clear the level. The energy shield is especially useful as it gives you one additional life when you're hit, and it regenerates every 20 seconds.