Crouch around, shoot, comms hack, disguise, repeat.
Soldiers and Operatives are supposed to cause havoc, but only the OP can easily get behind enemy lines. On SecTow, I always remove the cover off the gate controls, plant the bomb, switch to an OP, disguise, and then run up to the soon to be open doors of the hacking objective. But for each match, it depends. If there are no OP objectives, I'll just throw some caltrops and a sticky grenade down at their feet.
It takes some practice. Being a medium or a light body type helps, as the smaller you are, the harder you are to spot- and hit for that matter. Fat Spys are a no-no. I'm a double-speced heavy soldier/operative, so I haven't played the Operative as much.