Whenever you are going to this place for a mission trip or whatever, bus is the easiest way to travel. Rent a car is not advisable due to length of travel in the danger zone. You could, but remember to follow another car travelers, we call it caravan. There is only one way out to go from Cotabato to Davao or Davao to South Cotabato. Traveling time is about 4 to 6 hours due to bus stops.
Another tip, when you want to go to a public restroom, get ready with your loose change of philippine peso, you have to pay a maintenance fee if you want to go #1 or #2. Number two is more but it's nothing really for your money. No toilet paper either, you have to pay another for it. They usually use water not a toiler paper.