The insurgents melt away after the fight. There is no uniformed army to seek out and destroy. IMHO you will need to expand the punishment to the family of the terrorists. Sort of how Israel will bulldoze the house of a suicide bomber. Some greater price needs to be paid than simply killing the terrorists. After all, in their world view, you are simply giving them the only surefire way a muslim can enter heaven, via martyrdom. Bring the pain to their families. That type of punishment can be done via martial law. He could also borrow a page from our Marines. We used to bury the dead in the Southern Philippines with a slaughtered pig. That seemed to dampen the Islamist’s desire to fight back in the day.
The thing about Duterte is this -— He looks like he’s trying to distance himself from the West.
Trump invites him to come to Washington DC and he goes to Russia to meet Putin instead ( he had to cut short his visit because of what happened in Marawi ).
He’s very angry at the West ( EU and the USA ) for calling him put on his so-called extra-judicial killings in his war on drugs.
Yet, he needs military support ( arms, planes, etc. ) to fight a Muslim insurgency in the south. So, where does he go for help? Russia!
Go Figure...