This is a Metal Gear Solid 2 achievement.
To beat an enemy unconscious, you have to use punch punch kick combos and throws. To do a punch punch kick combo, unequip your weapon and press the melee button three times in fast succession. To throw, unequip your weapon and press the shoot button while holding the left analog stick to any direction. A clean way to beat someone unconscious without knocking him down to the ground multiple times is by punching a lot of times without doing the kick. You can do this by pressing the melee button successively, but not too quickly. If you get the timing right, the enemy will be knocked out after a few punches. Unconscious enemies are identified by flying stars over their heads. Do this to 30 soldiers and the achievement is yours.