This is easily the most convoluted of the character specific achievements.
You need to hit a single enemy with a basic attack while in each stance.
By default you start in normal stance, with just a sword.
Then you have your 1; Shield Bash. In this stance you have a sword AND shield.
Next is your 2; Bludgeon. You get a giant hammer/mallet.
Lastly your 3 which is Scourge. You get a flail-whip.
Your ultimate/4 'Eagle's Rally' is NOT needed but it can help.
The ideal way to go about getting this is to focus a squishy or already weakened enemy.
Either run up to them and hit them with a basic or initiate with Eagle's Rally which is good for doing some damage and stuns them giving you the chance to hit them easily with your basic attack from your regular stance. After you've hit them with regular you'll want to pick any of the three other stances to hit your opponent with next. Continue to cycle through until you've hit them with all four stances and then kill them.
I was told you only have a small window of time to get in all hits and then the kill, so don't get distracted.
It is not required for you to hit the enemy with the actual abilities when you switch the stances but they can help (alternatively they can also work against you as you might kill your opponent before getting through all the stance). If any of your stances 'fall off' and go away, then you have to restart all over again, it will go away if you don't hit anything with basic attacks. That said I don't know if you can just keep cycling your abilities.
It doesn't matter how you kill the enemy just as you're the one that kills them after hitting them with all four stances.