Xerath V Rengar

Okay so I have been playing a lot more of Xerath recently and with new Rengar coming onto the rift I am noticing a problem. Since Rengar can now avoid all forms of cc at 4 stacks which he gets after ulting and towards the late game his ult is almost always up. Xerath can't stop him because the only way for Xerath to block Rengar is by stunning him, Zhonyas, Flashing or GA and the only one that is reliable and doesn't have long cd's is his stun. So I was wondering if anyone else had any advise because relying on your team doesn't work when he 1 shots you upon contact.