Project Image: as in "hardcore rules", I recommend to use neither Project Image nor Insect Plague and also not abuse Simulacrum.
Initial defenses: I think that spellcasters could have their inital denfenses and initial summoning before the battle (but those spells have to be cast). Or we should give the warriors some round penalty to sharpen his sword and take on his armor.
![Image](http://www.gamebanshee.com/ubb/wink.gif)
Limiting items: it is really not realistic to compare pure characters, but we should somehow limit the available items (otherwise any mage could "counter" a sorcerer saying he have collected a lot of high level scrolls). I recommend that only quick item slots could be used. It seems we have agreed upon that no 9th level spells/scrolls are allowed. Now I would even say that no 7th or higher level scrolls can be used.
Flail of ages: very good point. The additional damages "hit through" stoneskin and disrupt spellcasting. An improved hasted character could hit about 4 times per round which cause 12 separate minor hits. Neither single specific protections nor even Mirror Image help, just give a little chance to cast (by catching some of the damages).
The best defenses are either Protection from Elements + Protection from Acid or a Protection from Magic weapons. Mages should think about memorizing those before a specific duel, but a sorceror probably haven't learned these spells. Maybe they could keep a scroll, but that could be breached. I wouldn't say that now a pure fighter could win against a Sorcerer. But this possibility may shift the balance in favour of mixed characters (fighter/mage, ranger/cleric, mage/cleric).