Natives
the number of the natives in a province increasing by 300 every month till they reach their maximum number, usually its a waste to kill them becouse 9/10 times they attack after you fail to increase/establish a colony/TP there , so you can add a colony and then movesome soldiers in to defent them.The number of your soldiers + your land tech points are crucial in the battle, for example a large army can eliminate all natives at once (waste)while small armys can only won a battle killing 1000-2000 of them.
For the provinces with highly ofencive natives i follow a cheating strategy; i first wait till an army of another nation clean the area and then plant a colonist in (even if the neutral army is still there) , if you succeed in colonization the foreign guys will turn their flag to white.
Early on eampty of natives areas are reunion islan and the island next to it, other 2 islands a bit north of it in the indian ocean, st elena in the central atlantic , fawclands, terra de fuego, at least 1 area in eastern coast of africa (somalia usually), tahiti (always earn a COT there) and tasmania.