I don't really know the circumstances of your game, but who's to say you're actually meant to fight the dragon? If there was a 100 foot tall frictionless pole in your path, would you complain that you can't climb it, or would you just go around it? I agree it's bullshit when a DM insists you must fight an impossible fight, but when you have other options, it's reasonable that some things are beyond your powers.
And it seems like you probably have other options, considering the fact that you seem to be able to go out and buy several doses of some obscure magic poison, so it doesn't sound like you're trapped there. (And, reasonably, you should have trouble trying to buy enough cockatrice grit in a short enough timeframe to do the job, but it sounds like your DM isn't one to limit abuse of magic marts).
And if you do try to go up against the dragon, no one here is saying you have to face it in a fair fight. But the opposition is more to you exploiting the system, rather than exploiting the current situation. Anyone can run Pun-Pun with a sufficiently lenient DM, but most people would consider any victories gained by such a character to be cheap. A good plan should be something that, even if DM doesn't allow it or it doesn't work, makes people think, "That was a pretty cool plan". The retelling of the story of how you implemented your plan should ideally require very little rules explanation for anyone who doesn't know the system.
Finally, something that bothers me a little personally is that your plans don't seem to really involve the rest of your party at all. If this is a recurring theme for you, I would discourage it, not only because most people don't like it when they have to sit back while someone else does all the cool stuff, but you lose a lot of strength when you make a plan as a member of a team and only count on your own.
Fine, I've decided to give the dragon a fighting chance and not cheese the encounter with Cockatrice Grit.
Instead, I'll just go solo toe to toe in melee with my wizard. Macho man, drop down, knock out, winner takes all.
Pre-fight: Mage Armor, Heroism, Protection from Evil, Shield, Reduce Person, Long Arm, Resist Energy: Fire, Protection from Energy: Fire, Fly, Maximized Mirror Image, Haste, Displacement, Extended Calcific Touch (cast using
Dweomer's Essence)
Surprise Round: Dimension Door 10 ft. away from Dragon, surprise round action Calcific Touch attack vs. Dragon.
Round 1 onwards: Move Action Fly until 10 ft. away from Dragon, Standard Action Calcific Touch vs. Dragon, Swift Action Shift (Su) school ability 25 ft. away from Dragon.
Combat should take a maximum of 6 rounds with this strategy. I predict an opening blast of fire breath for 0 damage as it's absorbed completely by Protection from Energy, then a bite attack in round 2 with 1/8 chance of hitting due to mirror image followed by 1/2 chance due to Displacement, etc. If combat goes poorly, I'll Dimension Door away, buff back up and return. I plan to keep an extra casting of Protection from Energy and Maximized Mirror Image available. All legitimate, rules legal, no funny tricks used, straight up fight.
Sadly my party is completely incapable of dealing any significant damage to the beast, with a Rogue/Shadowdancer, Oread Monk and a Rogue/Fighter being the remaining members of the party. They'd have to roll 15 or higher to hit this thing's AC, and even then they'd die from more than two melee hits from it.