I obtained this easily in Firefight.
Simply load up a firefight game and set yourself to invulnerable.
Now simply assassinate 10 enemies to unlock the achievement.
-Aim low on the back of the Covenant.
-You can Assassinate Grunts when they run away
-You can assassinate Jackels by meleeing them to break their shield and stun them then quickly move behind them and assassinate.
-For Elites simply get close, wait for them to try and melee you and move out of the way and then assassinate them from behind.