Syndra's ult and counterplay
Why just not make it a skillshot?
Have the target work similar to a karthus q and after chosing the area syndra channels for 1 to 1.5 seconds while the spheres hit that area. This way you are not guaranteed to get hit by all 7 just because she has 7 out. It might need a slight damage buff then but that is for another time.
While she can still flash ult it will at least be semi avoidable.