General Kyong is the boss after you enter the mines.
Here are some tips for the battle (played on Delta difficulty):
You start the fight with no weapons behind a crate. On the crate is a Submachine gun and a ammo box. Grab both immediately and go into cover.
Kyong fires a lot with a Minigun. I also strongly recomment tagging him with the binoculars, so you always know his position and where he is facing.
I did the stealthy approach, cloaked and sneak around him, hit him a few times in the head with the SMG, cloaked again and moved cover.
If you go upstairs you will find also a precision rifle which will help a lot since he can take quit a few hits.
Sometimes he will cloak himself, but he won't move too much, so it shouldn't be a problem. Just keep on the hit-and-run tactic until he gets down.