The real Sulyvahn’s fast slash is the one to parry. Stay close, move left and turn right when the clone harmlessly goes through you while it slashes. Hit the parry button immediately after turning right, before even seeing real Sulyvahn move. Max. difficulty level 5/5 and example 03:34. If you don't have a long bow, you can get one by going past have and unlocking the door (master key), or you have to go to Andre in the parish and head to dark root basin. There is a longbow with the leather armor set in dark root basin before you get down to the hydra area. I know, this is noobish way, but if you really can't kill him by use parry riposte or backstab strategy, you can try this one. This strategy can be done even NmFM.