Hardest fights in the game are the one in Fort Joy : the crocodilians, the frogs, the arena, Knilles, Alexandar, Voidbeast in the woods, necro witch in the cavern.
After that point, you scale really hard, and you unlock source spells.
During Act2, the game switches from "I need to survive Fort Joy" to "I need to become GOD". If you understand the game mechanics, you will not cheese a lot anymore, you will just unleash hell.
But playing classic and saying "I don't like tactician because it's bad difficulty design" while still wanting more challenge is a bit... silly ?
I only play on tactician and I feel like it's the way the game was built around, I find it really easy already so I can't imagine the bore you feel when you play on classic.
I personally find it stupid to search for balance in this kind of game : I mean the main goal of rpg is to become fucking op by finding the perfect stromp team.
But really, don't play on classic if you really want that much to use some tactics from time to time. Classic is for discovering rpg for the first time in your life, thats it.
Oh but I do agree about the becoming op. The key word is 'becoming', though. With a full phys party I felt I was too powerful right at the beginning. Warfare/necro/summons/ranger is just raw power that tears through enemies without much thought. And I like to think ahead in this game - actually consider the initiative order, remeber which skills are on cooldown etc.
I went with 2 phys/ 2 magic and I'm having much more fun now. Lvl 3 currently and crocodiles, frogs, griff and arena were all really fun and not too easy, but I still felt like everyone contributed (unlike a 3 phys/ 1 magic comp).
The only strange thing is that for some reason, this time enemies mostly tend to target my necro tank and my 2H dps, which is obvoiusly exactly what I want. They used to be more unpredictable in my previous 'test drives'. I'm not quite sure why that happens.