in act 2 when you get to the city you will find enemies called punks kill fifty of these and the achievement will pop.. easiest way to kill these was using a gun and aiming for the head will down them in 1-2 shots that way.
most punks use pistols and will drop pistol ammo a few in special areas of the game will use semi automatic rifles.
if you don't get it in act 2 there are more human enemies in act 3
first group is called jungle smugglers they appear near crashed airplane for night hawk quest you get from lighthouse in act 1
second groups of humans are on the main storyline missions Afran's soldiers and Afran himself
on prison island at end of game if you still don't have it will be a few bidf soldiers