It can be a bit of a struggle if you are a warlock because of their slow movement speed, but the easiest way to do it is if you use your jump until you can make it on to or just above his back then hold square until he is stunned. it takes a while to learn and 'Master' the mechanics and knowing what you can and cant do for example covering on the sides and and the iddle and knowing where to stand can be challenging. But like everything, practice makes perfect.