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Send in lesser (lower level) dragons first that fall for his ploys. Then the older, smarter, and more powerful ones make their push. Work it so a group of them working together make it to his inner sanctum, sucking up most of his contingencies. Before the last one dies, have it plant a marker or item of some sort that summons the elder dragon heading this assault. Then you can have a straight up fight.

As a dragon, I know too little about the capabilities of this Wizard.

I want to know what the contingencies are. I want to find loopholes and weaknesses.

I want to run a few feints, and learn from them.

I want to spy on the wizard and learn what tricks he is using to avoid being destroyed by enemies that he *thinks* are as powerful as me. I will then try to figure out counters to to the tricks he reveals. He'd better be resourceful with new ideas, because anything he reveals in an early fight I will have a counter for.

So, plans are two-fold:

1) Direct, brash assaults, lots of noise and power, with whichever allies I have that are up to the task, but disposable for the final battle. Ideally get one of the other dragon groups to waste its resources. Make him think this is the one, this is the big bad come to get me. Send a spy or scrying device along with the assault, get intelligence on what spells are being used, what attacks and defences need neutralising.

2) Deep cover. Get agents in places I expect him to be. Initially they should be regular unimportant NPCs - shop-keepers, town guards etc. Find out where he's heading, what kind of people he goes to for support. Get one of these "supporting" role characters close to him, gain trust, and report back whenever safe and possible. Deep cover = play it safe always, no back-stabbing or silly assassination attempts.

If your player is as clever as you make him out to be, he'll be expecting trickiness.  Of course any frontal attack will be a feint.  Of course you'll try to replace his allies and trick him into destroying himself.  He'll have mind blank and true seeing and death ward and protection from evil and all those other vital spells active all the time, and anything he can't hard counter he'll have a contingency against.

If the dragons really want to kill Anthedon, their best bet is to exploit the power of metagaming.  I don't mean they should use out-of-character knowledge--that'd be ridiculous.  No, their strategy will be to exploit the character's own expectations about how events will proceed.

What they'll do is start out with slobo777a's basic strategy of feints and infiltration.  So far, so good.  This is what Anthedon will be expecting.  As he counters the dragons' various assaults, however, he'll begin to learn that things are not quite as they seem.  There's a rogue group of dragons with similar, but not identical goals: while they aren't trying to thwart the Prophecy per se, they are trying to avoid the destruction of the world.  His primary antagonist among the dragons is being controlled by forces unknown.  The Lords of Dust believe that thwarting the Prophecy will shatter the locks on all of the Rajahs' prisons, and are offering him subtle aid and guidance.  The section of the Prophecy that foretells the destruction of the world may actually be a forgery.

All of these twists have, of course, been made up wholesale by the dragons.  The idea is that Anthedon will notice something subtly amiss somewhere, investigate, and eventually piece together enough clues to come to one of the conclusions above.  The clues should be subtle enough that he can afford to miss or discount some of them, and placed in a variety of ways so as not to give any indication that they all come from the same source.  In any case, depending on what he finds and what he believes, Anthedon should eventually come to believe that he has uncovered the "real plot" and act accordingly.  

The dragons have three goals for all this twistiness: 
1. To delay and distract Anthedon, giving them more time to prepare
2. To make him doubt his cause, and perhaps consider compromise
3. To get him to seek out a dragon (any dragon) as an ally. 

This last one is the most important.  The whole plot revolves around Anthedon thinking he's the active player in all this.  If a dragon comes to him and offers its services, he's going to be suspicious; but if he seeks out a dragon himself and after a lengthy debate manages to convince the dragon to join his cause, he'll feel like he's outmaneuvered his enemy.

This dragon alliance can take many forms, depending on the plot he pursues.  His allies might be the group of rogue dragons, his former primary antagonist (freed from the "real BBEG"'s control), or some other dragon that is willing to put aside their differences to combat the real threat.  The important thing is that Anthedon thinks he's in the third act when he's really still in the second.

In any case, he and the party and their new loyal dragon ally head off toward the "final showdown" with the "real BBEG".  This is a difficult and dramatic battle, and it needs to strip the PC of his hardest defenses, both magically and mentally.  Most importantly, it should make him expend whatever contingencies he has in place against trap the soul and good old-fashioned betrayal.  That is, one of his allies (but not the dragon) should betray Anthedon in a dramatic and shocking manner.  After those two things have happened, the "BBEG" kills the loyal dragon ally; with his final breath, the dragon gives Anthedon his prized magical doodad (like a powerful staff or something) so he can destroy the "BBEG" once and for all.

The doodad is, of course, a trap the soul trap.

Because seriously, what kind of idiot tries to pull the exact same trick twice in row?  (Evidently the kind that wins.)

The doodad-with-soul then teleports back to the dragons before the other PCs can break it.  The dragons can then break the soul down into its component XP, feed it to a demilich, throw it into a sphere of annihilation, or put it back in a body and make it hug a lavawight.  If they want to be a little kinder, they could also imprison it in Dollurh until the plane's Entrapping trait turns him into a shade.