The DK's armour class in Action difficulty is directly proportional to his level. Since the result is rounded to the nearest integer for display purposes, you will not see a change every level. There are some periodic deviations from a straight line for the other difficulty settings, but the approximate relationships I posted above are fairly close. On Very Easy, level 4 should give you a base armour class of 6 (levels 5 to 15 would be 7, 8, 9, 9, 9, 10, 11, 12, 12, 13, 13).
A(Action) = (5 * L + 32)/7
The DK's armour class is higher in Hardcore, but the monsters are also stronger. If the DK's armour goes up by 30% compared to Action, but the monsters are doing twice as much damage and you have less hit points, it will still be harder than Action. <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" />
Some resistances (for both characters) have a component that is directly proportional to a specific primary stat (ie a straight line, but again, rounded off in the display). Survival helps elemental resistances, Constitution helps Poison Resistance and Intelligence helps Spiritual Resistance.
On Hardcore, only some random encounter, non-plot related enemies are doubled. The guards in the starting cell block are are not doubled. There are pairs of ghosts in the cells close to the arena, rather than one, and I think someone said there were more rats, as well.
Your characters' resistances do not change based on difficulty level. I don't think enemy resistances change either, but didn't check. IIRC the different difficulty levels were added late in the design process, so AI should not be that different. The number of spells an opponent can cast is related to their magic points, so changes due to difficulty level will have an influence in that respect. If the most powerful spell they know takes most of their magic points, it will be cast rarely, but if it takes only half it will be used more. When an opponent decides to flee or cast a healing spell would depend partly on their hit points, so the maximum vitality would effect that.
One of the peeks of the week described a bug where boosting an imp's magic points greatly improved their attack performance;
Beyond Divinity Peek of the Week #22.
