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The easiest spot to earn this achievement is during Server S04_TALONPIKE in Network 1A. Once you are dropped into the server, you should be able to see two Defense Units patrolling directly ahead; they will slowly walk forward with their backs turned to you. Once they are lined up with the stack of Red Unstable Crates in the center of the room, shoot the crates to knock them down and damage their health. Now quickly jump down from your ledge and gran any Crate that is nearby and toss it towards them. Shoot it. Iff you're lucky with the positioning, the explosion should finish them both off, thus unlocking the achievement.

NOTE: It may be necessary to use a Nuke Virus to lower the defense level of enemy AI within the server. Additionally, you can equip upgrades to your weapons that increase the damage output from Unstable Crate explosions. Lastly, it may be wise to first lower the enemy AIs health before shooting the Crates, though take note that doing so will separate them and complicate the set-up for the achievement.