Prodigy
I recommend doing this one on the first missions as it is the shortest and easiest mission to get it. If you have problems you can always use a Score booster under the Boosters in armory, but I say its unnecessary as you will have to get the Skull booster achievement anyways which will make this one much easier to unlock. I recommend using the Hallow Skull (Spartan only has a shield) and the Famine Skull (Dropped weapons have much less ammo. Now when the mission starts, just kill as many enemies as fast as you can possible, sweeping the left then the right. Use the Assault rifle on elite, and pistol on grunts. Unless either run out of ammo, just use the other one. You shouldn't have to pick any weapons up. (RB is melee)