My Robot ArmyG 20

To unlock this achievement you simply need to hack into 20 spectres during any match with you data knife. This achievement is cumulative and doesn't need to be accomplished in one match. To hack into a spectre you need to be behind them and press X on your controller once close enough to initiate the hack. Once the spectre has been hacked it will be under your control and will fight along side you until it is destroyed.