Ryze has trouble with people that can play aggressively early on and beat him before he starts to scale well. Caster wise, Cass is probably his hardest MU because of her ability to poke zone and be faster than Ryze if her stuff lands. Zilean imo is also hard early on because his bomb range matches Q and beats Snare range, so he can double bomb before you can snare combo get away. Later on though, he kinda mows down most casters regardless because he gets to go tanky and "be the Ryze".
General rule of thumb: See Ryze, go Cass (if you actually know how to play her)
You can avoid it with boots, it's just not possible to circle around it in time to achieve what you need to. If you're going to harass her but can't afford to get hit, you gotta walk around the blast while she gets to back up or waddle around.