Who are the worst characters to fight in Klassic Tower (Very Easy, Medium and Very Hard) for everyone?

For Very Easy no ones to bad for me. Even Kronika is easy.

For Medium (I use Sindel jump in strat) anti air characters are annoying. Noob Saibot always anti airs. Erron Black and Robo-Cop as well. Spawn sometimes because he will anti air into the slowest combo ever. And Sindel will anti air you with scream and get a crushing blow.

Very Hard. Similar to Medium but Kronika is hell on this difficulty. Anyone with long combos suck because they will block you at some point and hit you.

It's been a while for me, but IIRC Noob saibot (with his anti air), Sonya, Kollector, Jacquie are the ones that gave me trouble. It's worth nothing though that you can also roll different "variations" of some opponents which are more pleasant to deal with because the AI will make them use one of the specialized moves that are easier to exploit. I recall encountering a "bot centric" Cassie Cage that was much easier to deal with than her brawler counterparts.

As for Kronika on very hard, best thing to do is just go for Uppercuts or combo's the moment you see her go for either her Orb or "force field" special moves. If you have a character like Noob, Sindel, Jade, Baraka with a nice "long range" uppercut you can exploit it much more comfortably. Liu Kang also has a really nice 1-2-Fly kick combo he can use which also works on Wakeup against her if timed correctly, shoutouts to @doctor_protonic for figuring that one out. Overall though, Kronika on Very Hard is absurd and its unfortunate that the best strat is just "perfect uppercut timing". There may yet be some more strats to learn in this regard.