This is a very easy achievement, choose any Civilization on Chieftain difficulty.
Within the first 5 turns the game will automaticly make you a warrior unit, explore a little with your warrior unit until you find a Barbarian camp, your warriors will always win because the Barbarians will always have a -50% defense due to be being uncivilized :)
If you meet another Civilization before you meet any Barbarians, don't attack them. Chances are you won't beat them, just walk around with your warriors and search for a Barbarian camp. There are lots of them so it shouldn't be hard finding them.
This is one of the easiest achievements in the game and you are rewarded with an awesome 3G! :)
The first 2 minutes of this video will guide you how to do so.