It is absolutely not true that offense beats defense full stop. By far the biggest challenge I ran into on Unfair was avoiding the attax of regular mobs I took for granted on lower difficulties. Battlemage gets you there by itself on Hard in a way other Magi don’t.
Quite opposite to my Unfair experience - I've found any attempts of stacking defense on anyone but designed melee units, in my case MC pure Oracle and couple of pets, absolutely, utterly worthless. You'd just cripple combat effectivity of your backline archers/mages if you'd try to get their AC high enough to soak some hits from high-AB mobs on Unfair. And even Oracle with the dog was mostly ignoring AC (never went higher that ~50-55) and had tanked 4/5 of the game with the mix of Displacement + Mirror Image + Seamantle + everything else.
From my experience, at least one character with godly 30+ initiative to provide surprise round is much more useful, since most hard fights are done before they even start properly, with generous application of archery, Illusion CC/Wierd and Hellfire Rays. Not the case in early chapters, but those are really easy to overlevel with some soloing.