Very Simple, what you need to do to unlock this achievement is perform a 'Multikill', this is where you strike down 4 enemies at the same time.
It is pretty much impossible to get this with a one handed weapon or a bow, I used a two handed hammer but other two handed weapons should be fine. Go to an easy level like 'Wolf Hunt' get a group of 4 Tomte's or wolves following you, turn around, let them crowd around you and strike. If you got a multi kill it should state so just above you character and then in the top left corner awarding you with a Blood Bonus for your troubles and then hopefully the achievement.
I'm not sure whether this is a glitch/bug or not but I was only able to unlock this using the basic
Strike Attack, it would not unlock with the use of any gift (skill) attacks. I also don't know whether it stacks and awards the achievement for a 'Rampage' which is for killing 5+ enemies with the same strike.