This achievement is story-related. You have to defeat the Garoran boss to earn it. You shouldn't have many problems with him though.
I defeated him using my forged two-handed axe just by focusing on him and mostly ignoring the skeletons he summons. Just hit him hard and fast and he will be dead before he can summon too many of the skeletons. Swinging around a two-handed weapon should kill some of them anyway, even while only hitting on the boss. Just beware of his charge attack, which hits pretty hard. Try hitting him from behind to avoid counter attacks. I had to heal only 1-2 times before he finally dropped.
Following the freed Diasora pup after defeating Garoran will lead you to another ritual stone to rest at and most importantly unlocks fast travel which makes traveling around the map a lot easier.