So some friends and I have been having a lively debate about how someone might actually go about killing a great wyrm red dragon, and a few interesting points keep coming up so I figured I'd farm them out to the internet.

First some context:
We assume that we have a party of 4 level 19 adventurers facing off against the dragon in it's lair in a volcanic basin. The dragon is CR 22 and is therefore a challenging boss monster for the party. The party is aware that they will be fighting a gw red, but does not know the specific spells that it might have prepared. They have plenty of time to prepare and purchase items with level 19 effective wealth (880,000 gp). The dragon is not specifically aware of the party's approach, but is too smart to be caught completely off guard. The party stealthily ascends the mountain and encounters the dragon lazing in the center of the volcanic crater. They take a few minutes to get up some buffs, rdrink some potions, and plan they're attack. They have one full round as a surprise before the battle begins in earnest.

Some important stats for the gw red dragon:
Colossal size, space: 30, reach 20 (30 for the bite)
AC39, Touch 0, FF 39
449HP, SR 33, Immune to fire, sleep, and paralysis
CL19, quicken spell, empower spell
9th level spells: 2 known, 4/day
The party is not specifically aware of the dragons spells, so the DM is free to basically choose whatever spells he wants on the fly (excluding gate and high level summons cause that's just unfair)

So here are some issues we have encountered in our discussions:
The radius of antimagic field and prismatic sphere list a "10ft radius emanation centered on you". They also say it creates a 10ft radius sphere. Antimagic field even says that parts of the creature which extend outside the sphere are not covered. Does this mean that the dragon would essentially extend outside of the sphere by 5 feet in all directions, rendering them kind of useless? Or would these effects just extend 10 feet from the dragon's square, essentially creating a 50ft sphere?

How does rod of cancellation behave inside an antimagic field? If the dragon has concentric prismatic sphere and antimagic field, does the rod still effect the sphere? Is it suppressed? Or can one rod dispel the field and another dispel the sphere?

I am sure we had more topics but these are the ones I can recall immediately. If you come up with more discrepancies I'd love to hear them! I feel like fighting a gw red dragon is like a milestone in a tabletop players career, and I want to know how it'd go down.

Thank you guys very much!