Yeah the decoys are excellent, throws a lot of people off guard. The fully-upgraded Stun Arm is excellent too for taking out guards since that lightning strike has a pretty good radius of effect.
The defender should honestly just fvck off after the first death. The FOB can handle itself, ever since I've made all my platforms 3 and above with max security, no one has been able to infiltrate successfully. Having a defender there just makes it much, much harder than it already is.