when using spies, agents or diplomates make sure they get the "trained" trait before sending them out on missions. actually, once they are trained keep them in a developed settlement or capital for a few more years untill they're over 30 years old. i try to use them togther with my legions. before attacking i send my spy forward wreck havoc with my agent and after i raid the settlement i'd either sue for peace or gift it to some other faction to create a buffer zone.