Respect

As it says in the achievement, simply /bow to them when they bow to you once you have defeated them (they will stop attacking and go friendly), you have a few seconds grace time to do this.

It may be an idea to create a simple macro and put it on your action bar, and to track the achievement.

Another tip may be to "(Shift + V)" to bring up the name plates to help make it easier to see when one of the NPC's turn friendly.

This will then buff you with a "vanity" buff for 3 seconds "Respect".

It now also seems that when one player gets the achievement it becomes group wide and all receive the achievement.

So it may be an idea to get a group of friends to group with you and all try the achievement together with one of you simple /bow'ing, whilst everyone else simply dps's them down.

Good Luck and have fun !

This is a group achievement. Everyone will get it if one person does it.
/bow after you defeated one of those fighters.

To make this achievement a little bit easier, you can turn on your friendly nameplates (Shift + V) by default.

The mobs will become friendly with around 0-20k HP, will be easier to target and do /bow.