I started to play the game on "Challenging" difficult, and I experienced some differences already. Sadly I can not yet present a full list of fights yet (I started to make notes on them at zone 4 only), but here are some strategy-advices for "Easy" diffculty, and some more things:
I heared there is a maximum level restriction (lvl 30). I myself finished the game at lvl 25, but I made several training-fights (good for additional money and experiences).
Now on Easy most of the times relying on pure brute force will work, there are only two figths, when it is at least advisable to have certain tactical skills. One of it is against the Baron. He has annoying abilities (like the curse what makes you loose 10% or your health at every turn for 10 turns), but he relies heavily on using his focus. So take his focus away. The best part is, that his focus-regaining ability damages him at the same time, so spaming focus-stealing on him is the cheese-cutter solution. (All the fights on Easy are possible to be solved by just brute force, but that requires not a little luck.)
The second fight where you can have problems is Clemon (zone 3's last boss). He has a tendency to cast a curse on you what turns all healing you take turn to damage - and when you're in Toxican-form, that is painfull and potencially lethal. So having the same type of skill yourself and casting it to redo the effect can be a lifesaver.
Now there are two "achievements" that can be done on Challenging difficulty. The first one is obvious, and can be done easily if you have time to level up if you want, as you're allowed to do that
. (This is Black Magic for what you have to kill Clement w/o any teammates.)
The next achievemnt is called "pacifist". For this as I undertsand from comments on the topic is to not deal yourself damage by direct contacts on the Hydra, or any of the two FireClaws. But you can cast damage-after-time spells (like envenom). The Fire Claws are sensitive on ice, poison and electricity, the Hydra is resitant to those elements. You are also allowed to heal yurself or your party. You won't be blamed for damage your companies do either. For this fight do not forget that you are allowed to respec!
For "Heroic" difficulty there are 3 achievements. For "All Star" you have to beat the game with all three classes - but hang a second, do it with two builds what you don't like that much, then come back one more. When you beat Legend, you will finish the game also with the third class. Also unlocking "Legend" means you will also open along the way "Predator". So this means 2 less time you have to play the game, ok? (Also if you go for Legend try not to buy anything in shops, not spending skillpoints as long as you can solve things without them, as your resources will be limited, so you can't allow yourself any mistakes.)
The last thing I wish to tell here is the fights in Zone 1
0) this is actually the tutorial level. You can skip this, but don't do that if you try to go for the Legend title, as this gives you experience too.
1) prison guard
2) 2 prison guard
3) brutal convict + agile convict
4) brutal convict + cunning convict + agile convict
5) 2* prison guard + doctor leath
6) twisted experiment
7) the warden
8) Frankie + doctor hedger
9) zpci elite
10) Felicity
Now in "Challenging" difficulty in Zone 1 I had "problem" (aka brute force was risky) against The Warden. It hit hard - but he needed some focus to do that. So I suggest being able to null his foxus.