Trying to make a monster stronger then the Tarrasque, but how?

Quote from packman767 >>

So I think what would be really cool is to take a Beholder monster as a starting stat card.  Increase the hit points.

You might call it a Titan Beholder.

And in addition to its already difficult powers to overcome, including an anti-magic field, give it the power of Invoke Duplicity that Trickery Clerics have.

But give the monster one fatal flaw exactly.  It is vulnerable to non-magical ranged damage directly in the Eye.  And one minir flaw: And it cannot dispel illusions, but is easily distracted by them.

But of course tell the players nothing and let them figure it out.

 The Moon is a Beholder. Call it the Lunar Beholder.

I'm guessing it'll be stronger than a Tarrasque.

This is all way too complicated to actually run at a table - take the tarrasque double its hp and have it reflect on 4/5/6 let the damage go through on a 1/2/3 and give it vunerability to the special sword problem solved - maybe add +5 to its to hit and +10 damage and give it a range attack that does the same exact damage as the bite call it a howl - simple. 

Unless your playing epic levels thats a TPK for 95% of groups out there at l20 

Only solution you really need:  Make it fly.