Elex

You will get this naturally as you advance in the game. When you first start killing anything is a battle, once you hit level 10 things become easier. I was able to defeat groups of 3 from that level.

A great place is Valley of the damned right near that checkpoint, there are three to four groups of exiles that will attack you. Make a save then mop them up, keep reloading the save until you have the achievement. Make sure your companion doesn't steal your kills.

I highly recommend placing your early points into Strength, Dexterity and Constituion they will allow you to wield the best weapons and survive battles, focus on the other 2 later. I found the attack much more devastating than the RB and rarely used that button.

I found the Chainsword really effective against humans, if you modify it to become a Gore Chainsword it does massive damage to flesh.

Also Human enemies seem to be categorised separately from Reavers/bandits.