How Strong is Cass?

Hey, I'd like to start off by saying that I'm not very good at League so bear with me if I misjudge anything.

So basically, me and a couple friends have been arguing about Cassiopeia. I personally think that Cassiopeia is completely fine in her current state and has places where she excels. Because I do not play Cassiopeia or see her a lot in my games, I can't give a very accurate overview of her kit, but she seems a lot like an AP ADC(I hesitate to say APC because that term could be used for basically all mages) who specializes in high, sustained single target and tools to stop help with her kiting and self-peel. To me she seems like a high micro champion who has a fair couple things to focus on. I think that the focus on reapplying q while kiting to maximize damage takes a fair bit of skill to do properly and the fact that Cassopeia's damage is output at at least a moderate rate compared to other mages is what balances her.

My friend has a very different opinion to me. He thinks that Cassiopeia is a "broken" or low skill champion. In his opinion, Cass is overpowered because she has a highly spammable e that does, in his opinion, too much damage. He basically stated that Cassiopeia is broken because she can harass her opponent pretty well without having to apply much effort. To prove this we did a 1v1 with a friend of mine jungling to manage my side in case anything went wrong. He played Rengar, I played Kalista and, the friend I was arguing with played Cass. Basically, I got pretty much destroyed because of my own mistakes and the fact I wasn't very experienced as Kalista. The game got to a point where we had to 2v1 him at which point he simply declared that we only had even a chance because Rengar and Kalista have synergy. He stated that junglers can't even gank Cassiopeia because she can even 2v1 easily with her ult.

One thing I noticed during the game is that he bought three items that really damned Rengar and I. He got and which are what made him really hard to fight. He basically shut down our mobility and used his items to nullify a fair bit of damage and good on him for doing so. I think that the only reason that he thinks Cassiopeia is overpower is because Rylais makes her extremely hard to escape her once you get hit by something and because Archangel's into Seraph's works really well with her kit.

So what I want to know is what do you think of Cassiopeia?

Sidenote: Anyone have any tips for playing Kalista? I really enjoyed playing her because of her passive and I really feel like picking her up for fun. (I know that Kalista is super off meta, and I understand why she's in the state she is right now, because I started playing league last year when she was top of the meta)