Well, there are only two good ways of doing it: the more diplomatic way and going full exterminatus. Diplomacy is possibly the more succesfull method but exterminatus is just too much fun.
My first GOG-game was the Witcher 3. It also happened to be my last pre-order. At the time of writing I'm trying to convince myself not to start a replay of it right now (just waiting for the next-gen edition...)