To unlock the Protect the People trophy in Devil May Cry 5, you will need to defeat all of the enemies, including the one that is hidden, during the game's first level.
The whole level takes place on a bridge and is very linear with most of the enemies being very easy to see. The one hidden enemy that you will need to take care of appears around halfway through the level when you drop down the broken bridge following a cutscene. Turn around 180 degrees and you will find a Red Mantis guarding a purple orb. Kill it and continue on with the level taking out the remainder of the enemies you come across.