Querente's page

You can do it like special class abilities DC.
Instead of relying on the spell level, you take half-level of your character.

Spell DC = 10+Half your level+relevant stat.

Then spell level has no importance anymore though, but the same spell cast by different level casters have different DC and different damage values (assuming the spell scales).

Just wondering if you should count only caster-levels or all class levels for this.

I am not sure if there is really a problem.

If we look at level 20, we have
Good Save : 12+Stat
Bad Save : 6+Stat
Spell DC : 10+Spell level+stat

The stat is an attribute bonus. The wizard uses INT and the defender uses Dex, Con or Wis as bonus. Assuming they can be equal, we can remove it.
In addition, at level 20, highest spell level = 9
Good Save: 12
Bad Save: 6
Spell DC: 19.

This means, the good save person needs to roll a 7+.(70% chance?)
This means, the bad save person needs to roll a 13+.(35% chance?)

Wizards has feats that increases the DC.
Defenders have feats to increase the resistance (Iron Will, etc).

In addition, the defender can get a cloak of resistance of +1 to +5 for extra defensive bonuses (prolly countering the wizard +Int bonus from magic items).

Then there are evasion class abilities that lowers damage and even mantle of spell resistance to force the caster make another chance roll.

I am not sure what items help a wizard to increase his DC besides +INT bonus items. I know there are items 'of Wizardry' but can't remember what they did again.