Chivalry Medieval Warfare

This was easier with a couple of friends.

Have two friends playing on the enemy team, playing as archers (without any shields! I can not stress this enough.)
Pick a vanguard with a two handed sword with high damage, to play as yourself.

Find a quiet spot for all three of you to hide.
Give both of your friends a poke with the sword first, better chance of taking both of them out in one sweep.
Have your two enemy friends to stand next to each other and make sure their necks are level.

Aim for their necks and swing away, now hopefully your aim is true and you'll grab yourself an achievement.
You may need to try a couple of times before you get this, but just be patient and following this technique you'll net this.

Now swap places with your friends and help each other out.