Ancient Red Dragon, SR 28.
Caster level 12.
Need a 16 on the die roll minimum to match the SR. So roughly a 25%, although if you've taken Penetration feats, that boosts a slight bit.
I'd be more worried about that 34 DC fear aura though. Will+1(Rogue), Will+4(Wizard), Will+5 (Arcane Trickster). So about +10 from class levels. You're going need some respectable resistance help even with exception stats and a +6 resistance cloak. Suggest possibly trying to get a Remove Fear or similiar cast on yourself before going in there.
Fort save dependant spells are hard, +28 Fort Save (29 minimum save effect, and you don't look like a DC min/max archmage freak so most likly no contact). Will saves are at +26 (27 minimum. another unlikly to be able to get DC that high without the Tatoo Magic redundant focus). Reflex at +19 (20 minimum) is your best bet and only one likly to be able to break their saves at all without Save Focus. Downside is that Reflex spells lack Save or Die effects.
Acid Fog might be useful except for Ancient Red Dragons having Wing Buffet.
Antimagic Field is just simply a bad idea, it opens you up far too much to Flyby attack pain and most of your defenses negated.
Geas might be worth while, although that's a long term effect, and the secondary effects are Fort save (ie, they're going ot be made).
Chain Lightning will be able to deal damage, but will only really be able to splat with the Primary Damage.
Otiluke's Freezing Sphere is good, go with Cold Ray (if you can manage to get it flanked, you get bonus Sneak attack damage, and their's no save, and cold damage). It is a ranged touch, which nets to about AC 6 for an ancient red dragon (12d6 fire subtype doubled to 24d6, it's something), better suggestion than going with Chain Lightning.
Displacement, for obvious reasons.
Downsides I see is you just not being about to dish out enough damage before depleting your spell resources to kill it, even the 24d6+7d6 from sneak attack averages 108 or 108 points of damage, and they have around 527 hp.
Note about impromptu sneak attack is it actually bars getting bonus damage (no crits, no bonus damage, etc). [ Edit BAH this is wrong, misread the stupid line ]
Blindsight = True Sight without the spell and not bypassable but doesn't see Ethereal.
Ancient Red Dragons cast spells as a 15th level sorcerer, so possible.
LMAO Orco, adding that to my sig now

too funny.