I think of a few directions for balancing combat.
1. I'm not 100% sure about this one because I don't know the formula. Do critical hits/crushing attack/stunning attack depend on the enemy's level? They really need to be. Otherwise monsters late in the game will have unjustifiably high critical hits etc against you, which make their power unbalanced.
2. The resistance formula needs to peak out at the proper place. With the current formula, it seems to reach its peak at around 40%, which is too low. What's important to figure out is where's the realistic curve: what is the level of max resistance per level; what is the min resistance; what's realistic to expect someone to get with resistance-boosting skills and high spirit; and to tweak the resistance formula to be around that curve.
3. Resistance rewards from spells and such almost certainly need to get higher and higher per level to match the diminishing return found in the formula.
4. I think plate armor might be suffering from the same problem in the armor absorption formula: it only slightly increases your armor at a level where the diminishing return is such that a slight increase doesn't make much of a difference at all. Again, I think it's important to look at realistic armor levels for different classes at different levels and make that curve be useful.
Essentially, I think we're seeing the downsides of the new formulas for resistance and armor absorption: high level characters (meaning those with relatively high armor or resistance in the curves) suffer because they can't improve their resistance/armor enough to make any difference.
The solution, I think, must come from using real data to improve the curves. While monster damage seems easy enough to predict per level, it's really hard (at least for me) to figure out what kind of armor level different characters have at different levels, or how much damage each character can dish out. I'm hoping the character repository can help with that. So check in your characters to the repository guys!