"Complete an Operative Primary Objective within 5 seconds of breaking disguise"
Start a private freeplay game, set the game mode to objective, map to security tower, set the difficulty on easy, set min/max team size to 1.
You'll need to play on the side of the resistance, start as a soldier, rush to the conduit and set up the charge. When it goes boom switch to the operative and proceed to hacking the safe, start the process and continue until it hits 95% killing the hostile whenever he shows up. When it hits 95, cancel hacking and use the corpse to disguise yourself and then complete the hack.
And there you go, nice and easy, in just under two minutes.