In the last mission in "Motuyoc" region you must enter a mine and kill Carl Bookhart. But before you get there, on the construction site there are a few enemies. Deal with them by using synchronized shot.
When you are inside you must slowly and patiently walk into the depths of the mine. There are a lot of enemies but they attack individually or in pairs which makes things much easier. Also, there is no time limit so you don't have to be in a hurry.
When you get to the stairs go down where the last group of enemies waits. Among them there is this mission's target - Carl Bookhart.
The completion this mission ends your hero's struggle in "Montuyoc" region.