## Take away his magic.

A sufficiently paranoid Abjuration wizard with nothing but Dispel Magic, Anti-Magic Field, and Mordenkainen's Disjunction prepared in every spell slot can shut down almost any kind of magical protection - and if it can't be shut down, it can probably be made a whole lot less reliable.  A Truenamer with Thwart the Traveller and reversed Spell Rebirth can also pull this off, as can a psion with Dispel Psionics and Null Psionics Field.  Once you've got your spellcaster trading actions with the dragon, your fighters close in and kebab him.  Considering I've seen optimized charging builds deal well over 1,000 damage in a single round, you don't need that long of an opening.

## Ask the Mailman.

There is a type of build well known as the Mailman ([linked here][1]).  Long story short, this build uses direct-damage spells with no spell resistance and long range in combination with metamagic feats to deal absurd amounts of damage in a way that is extremely hard to resist.  Short of just not being in the mailman's line of effect, there's no easy way to avoid taking several hundred points of damage from a build like this.

This can especially be combined with an Abjuration wizard as above, but that may be approaching your god-slaying restriction.

## Catch him unawares.

Though magic can stop any threat, it's very hard for it to stop every threat.  At some point, there is going to be a weak link of some resource that is stretched a bit too thin.  

For instance, this dragon is going to need to rely on casting Detect Poison, lest he be trivially subdued by an assassin sneaking something into his food.  This means he either has to be constantly using spell slots on it, or he needs a wand that can break, be stolen, or run out of charges.  

This dragon also needs constant protection as he sleeps.  He needs to regularly recast magical wards, maintain the traps around his hoard, and keep his fanatical kobold cult in line.  These things take a lot of time and effort out of a dragon's day, and many may require constantly expending spell slots or wands.  A single sabotaged trap, bribed minion, or uncast spell can result in a red dragon who never wakes up.

The dragon also can't take a lot of these protections with him on raids against his enemies.  Once a dragon leaves his lair, the lair can be overrun by enemies or sabotaged in any number of ways.  Slap some Nondetection and Mind Blank on a rogue with Darkstalker and a +40 bonus to Hide and you've got a deadly surprise awaiting the dragon when he comes back from his princess kidnapping escapades.  The only thing that will protect the dragon at that point will be Contingency (assuming he hasn't already burned it) and his own hit points.  Since most contingencies rely on some form of "get me as far away from here as possible" effect, all you need is two assassinations orchestrated back-to-back.

Even Contingency goes away if you give your assassinating rogue(s) some Boots of Temporal Acceleration and a scroll of Anti-Magic Field.

## Diplomatically.

Use your imagination here.  The dragon must be motivated by something - whether it's complex political intrigue or just simple coin.  Find a motivation and either exploit it to get close enough for the kill, or use it to aim the problematic dragon against a bigger enemy.


  [1]: https://forums.giantitp.com/showthread.php?447435-quot-The-Mailman-A-Direct-Damage-Sorcerer-quot-(from-Wizards-forums)