Of course the big exploit is to give away your own core cities to your enemies , then you get a free stack of troops when it revolts , or you recapture it if it doesn't revolt and exterminate for the money and population reduction.

I was Egypt's Protectorate in one game which allowed me to spend a long time assassinating and sabotaging their cities, eventually I only had an odd battle or two to completely eliminate them. I had little trouble getting their cities to revolt.