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Before heading into a Prime Unit Server, go ahead and access the Augmentation Menu. You'll want to spend one Praxis to unlock the Titan aug (there's no need to upgrade it). Now stock up on your most powerful weapons and gear (Expansion Items that increase Energy would be beneficial, too) and head into Server S06_TALONPIKE in Network 1A.

Once you gain control of your avatar, use a mixture of your weapons and the Red Unstable Crates scattered around the map to dwindle the Prime Unit's health down to a quarter or so. Once the enemy is hanging onto a sliver of health, activate your Titan aug and finish it off by any means. So long as your aug remains active during the kill, the achievement will unlock.