This is the same as the similarly named achievement from the original 360 version.
Here is the original solution from NahteLedana.
This method involves no serpent holes and is a lot quicker then all previous methods posted above, go to the crossroads and kill all the enemies on horseback, after this rather then entering the serpent hole go to the library which was previously guarded by the big troll, enter here kill the enemies and then walk back out into the streets all enemies will have respawned and makes killing on horseback a lot quicker!