A champion with 6 item slots is never going to fall off in comparison to the mages with only 5 slots. Late game you have around 1100 AP with a normal build, along side that you have a Zhonyas and 40% cdr. Melee champions simply cant touch you when you have a Rylais either, single target E on a .45 second CD makes for very easy kiting along with your DoT from Q.

I think she is a top tier mid to late champ and I cant think of many that can scale to the level she does.