Too Human

a polarity enemy is one that is a different colour to the rest.

The following link is not my work but gives you a better understanding of what a polarity enemy is and all of the different ones out there.

http://toohuman.wikia.com/wiki/Enemy_Polarity_Index

All that being said, it's impossible to beat the game and not run into a polarity enemy.

You will unlock this achievement early into the game.

It is best to kill them with your ranged weapons as killing them at melee range usually results in a DOT (damage over time) debuff on your character or in the case of the frozen ones, will freeze you in place until you are hit by an enemy.

*Solution added only because the original solution was very vague and didn't even touch on the main strength of polarity enemies or their effects.

This achievement can be earned earliest on the first mission. To defeat a polarity enemy is to kill an enemy with red(fire) or blue(ice) armor. This will score you 5G!