Kill one they find another one. The money keeps flowing in as they control more territory. You have to take the territory away from them. To do that you also have to kill the ones in that territory.

The faster you do it the better. Lighting war is the way to go attack their territories all at once a combination of air, tanks and troops.

Can get it all done in a month or less if we have what we need to do it. Thrump is right about that.