The main battle Duterte is fighting is not against Islamic insurgents; it's against drug dealers and all the other corruption which is woven into the fabric of an incredibly corrupt country. The Islamic rebels are small in number and are relegated to a few jungles in the southern part of the Phils. They are no threat to the vast majority of the country.
The Philippines is a very poor and over-populated land, perhaps the poorest non-black country in the world, a place where guards with assault weapons are ubiquitous at not only banks but shopping malls and fast food joints, practically every business small and large. Duterte made his name as the mayor of Davao, the only semi-well-run city in the Philippines, one where criminals were treated with the same iron hand he is trying to institute throughout the country. He is a true populist revolutionary, crude and roughly hewn, but well intentioned. He may or may not succeed as he is facing the same entrenched forces that Trump is in the U.S. (or would be if he wasn't a snake oil salesman), similar to what the late Hugo Chavez was up against in Venezuela and what left-populist governments are trying to do in Bolivia and Ecuador, and of course as a result like Duterte are also on Washington's **** list.