This is an odd achievement, it's description is wrong.
Go to an area where the enemies respawn (I went to an area with 3 oak enemies)
Stand in one area (ideally against a wall so you can't be knocked back) and then keep killing the enemy's with your sword (you don't need to get your spirits to kill them).
After a few minutes you should get the achivement, some say you will get some dialog as well but I didn't. You also might not need to level up to get achivement as I didn't but I spent around 5-10 mins just spamming the A button until it popped.