True, SR applies, but it applies individually to each layer >:3
six out of the seven layers are capable of affecting a red dragon (fire damage, acid damage, electricity damage, death, stoned, insanity, planeshifted)
That means six chances to make a caster level check every time the dragon moves or is moved through the wall, including each time it tries to attack through the wall with its natural weapons. All we need is, say, the insanity effect and one damaging effect beat it's SR- 40% chance the dragon will attack the caster (who will be on the opposite side of the wall) or closest target (10% caster and 30% closest target) 30% chance to do nothing but babble incoherently, 20% chance to run away from the caster, and 10% chance it will actually act like a dragon... if the closest target or spell caster is always opposite the wall from the dragon, it will make attacks through the wall and be affected by the layers it failed it's SR on.
with an SR of 28, my wizard's caster level is a minimum of 15 to cast the spell, and the spell penetration feat; that's a +17 to the roll to break SR, requiring a roll of 11 to succeed, which is 50/50 odds of any given effect penetrating the dragon's SR. Three of the effects instantly neutralize the dragon if he also fails his save (which is the difficult part), one of them reduces his combat effectiveness (again, if he also fails the save), and two of them deal damage directly.
The death effect is the best bet, though, because it does con damage even on a successful save, which lowers his fort save and HP and makes it more likely to cause instant death on subsequent attempts
--edit--
also, Polymorph any Object or Temporal Stasis, I should be able to cast them with a DC30 at that point (of course, the dragon would only fail the fort save on a 1... msot transmutation powers save against fortitude...)
an elder elemental would be a good sommon for summon monster 9- earth would have the mass and strength to bullrush a dragon in to a prismatic wall, a fire elemental would have the HP to last though the natural weapons, half it's own natural attacks are bludgeoning damage, and it would be immune to the breath weapon
the sorcerer can get polar ray as an 8th level spell, and have a minimum of +16 to the caster level check against SR, adn a minimum of 16d6 damage on a successful ranged touch attack, doubled for the dragon's weakness to cold damage...