To elaborate on the above post, you can do that one of two ways: the easier of the two - requiring only a bit of luck, as opposed to ninja-timing - is to grab and throw him - making sure not to toss him past the "Memory Inaccessible" shield of the battlefield - then switch to the Hidden Knife, walk over to him while he's geting up, and shank him.
The other way is to back away from him a bit, switch to the Hidden Blade, hold down R and wait for him to come to you. When you're equipped with the hidden blade, he'll actually attack you, giving you the oportunity to counter kill him; the downside is that, either as a result of the weapon choice or the general difficulty of this fight, is that your timing must be spot on to pull it off. Just hope he doesn't do his big windup attack, because that will almost always whiff, and pray that you push the X button right.
Once he's shanked, you're done.