The fourth time you encounter Master Kohga, he creates a Construct mecha robot to attack you.

Master Kohga: Ancient Construct Mech Robot

Whenever possible, shoot at Kohga to knock him out of the Construct, then hit him repeatedly while he is on the ground.

After a while, he will start putting a force field around himself. At this time, he might swing a punch at you, or put up a shield if you are too close, or he will jump to a corner.

If he jumps to a corner, sometimes he will point the Construct's arms in your direction. The arms will then shoot beams of electricity, and the arms will move so that the beams will close in on you, so try run toward the Construct and then behind it, where the beams can't go.

Or he will jump to a corner and create three large bombs, then send them toward you one-by-one. If you can shoot them from a distance, they will explode before they can reach you. You can use Recall on the final bomb to send it back to Kohga to hit him.

Another way to damage Kohga is to use a heavy weapon to knock the construct into the wall, but if you try to do this while the Construct is swinging its fist to punch you, it won't work, so run away from the punch first, then run back to the robot and swing your weapon until it hits the wall.

When the Construct hits the wall, Kohga will fall out, so you can hit him repeatedly before he gets back into the robot.