Does Anti-magic shell negate DR? Yes.
Does it matter? Considering that the dragon has a 40 something AC and over 700 HP, no.
If you can cut it down to just the fighters being able to hurt you, you're fine. At best, they'll do about 15-20 points a hit -- and only 2 or so attack per round from them have any chance of hitting.
Considering that they'll have a max ac around 25 or so without their magic, your dragon can just 20 point power attack them all death (don't forget to take cleave).
If it comes down to a duel, force a grapple with the poor bastard. You'll have like a +60 something on the check (base attack bonus + Str + a +16 bonus for being colossal). Come to think of it, the Snatch feat gives your dragon improved grab with its bite attack. As a side note, if ya want to grab someone but not spend all effort on grappling them, you'll take a -20 penalty. Bringing you down to a mere +45 or so. You can't bloody lose the grapple, so who cares?
You've got 10 feats or so to work with.. Fly-by (which WILL work with Harm spells, BTW; fly-by works with anything that's a partial action, making it much cooler than Spring attack), Wingover (turn around as a free action, which is VERY important for a Clumsy flier), Hover (between this and wingover, your hulking brute can fake having Good manueverability), Large & in-charge (sort of nice.. much better if your dragon is built with point-buy or something similar so that it has a positive Dex mod, in which case take Combat Reflexes to make it evil), Power Attack (a genuine must), Cleave (trust me, you'll use it), Multi-attack (maximizes guarantee of off-attacks hitting), and maybe something really bizarre.. improved unarmed strike & then Stun Fist to get Stun-bites that impose like DC 35 fort saves?
For the more magical route...
Time Stop, MIRACLE, Expeditious Retreat (flight speed of 400+ woohoo!), HEAL, Maze, Greater Dispelling, Mordy's Disjunction, Gate (get some bitchin' allies -- you can force a Solar's obediance with this puppy, and it doesn't have to be paid if everything dies in 19 rounds (or better yet, everything else AND it dies)), Horrid Wilting, Haste, Improved Invisibility (combo with Non-detection, if possible -- they'll need to pass DC 35 checks just to see the damn dragon), Sunburst (don't underestimate an area-effect blindness spell), Teleport, Iron Body (50/+3 Dr, +6 Str, -6 Dex, immune to lots of things, including crits), Prismatic Wall (grab people and throw them into it), Divine Favor (+6 Luck bonus to attack and damage; not bad for a 1st level spell...), Righteous Might (Colossal to Earth-shattering?), Destruction, Wail of the Banshee, Negative Energy Protection, Contingency (Heal), protection from elements (Cold)...
My own evaluation of the dragon's odds is that it can only lose if the player allows it to. Something that can kill the Tarrasque for breakfast shouldn't be prey for human scum.
Honestly, a great wyrm red or gold is closest anything in the system comes to completely unstoppable.
