you have to make him doe the strike where he slams it but dont walk up it. make it do it a couple of times but you might need to make him hit a small stone bit sticking out the ground thats a circle and when a small cut scene comes up then you climb up his arm go up 2 his head stab stab stab until the mark dissapears then drop down climb around stab stab stab on the other mark blech hes dead.

I always climb up the hand holding the sword and then up that shoulder to reach his head
first. After finishing with that spot, I just drop from his back and grab his lower
back/stomach on the way down.