The achievement "No Easy Way Out" specifies:
In Ch 1, Ep 5 of Spartan Ops, survive the enemy assault during the defense on Normal or harder.
I've played "Core" a half dozen times, in co-op, solo, on Normal, Heroic, Legendary, etc... I've also tried avoiding death in the final section where you defend the artifact, or even the whole Op, but still I haven't unlocked this achievement. What gives?