check this character profile. built with 20 point buy, which is reasonable for a ´boss´ IMHO.
dragon disciple doesn´t advance the non-draconic bloodline (IMHO), so I´m not sure about the cross-blooded thing.
there´s also eldritch heritage, but that´s kind of feat intensive...
I think I built a pretty damn effective build myself without any of that stuff.
Depending what level you are, using spells/scrolls to buff STR and CON can be a great strategy,
and let you spend wealth in other areas than really big bonus belts, etc.
I kind of built around a specific focus for an arena fight with fixed rounds for pre-buffing
(and so being able cycle Rage via Fatigue immunities so I can chug potions faster within Rage AND cast scrolls outside of Rage was a focus)
but without that arena focus, you can get alot more other abilities (feats/rage powers), along with using other races
(half-orc with the lucky tatoos saving throw trait instead of ferocity is a good one, dwarf is very good as well)
I also used the 2-Hander Fighter bit to make sure my single attacks hit as hard as possible (e.g. when Slowed, which happened in the arena fight) along with some extra Feats, but depending on your needs and the level you will build it as, you may just want to go full Barbarian. (at higher level you won´t have any lack of feats, and honestly without the need to rage-cycle for potion-chugging simultaneous with casting, you will have more free Feats/Rage Powers as well).
check the post history for the character if you want to see how he handled an arena fight (4v4).
let´s just say no enemies were left standing after a 12-15 round fight that started out with the other side winning Init and Casting Black Tentacles+Stinking Cloud on all of my team, affecting everybody except me (because of strong CMD and Fort Saves)
he was built without Ult Magic and Ult Combat, so there are even more options now if you have those...
as-is, he´s honestly very vanilla apart from the potion-chugging/rage-cycling strategy...
but that just makes him simpler to build upon.
(BTW: I didn´t give him armor spikes for some reasons, which i regretted during the end of the arena fight...
(using a reach weapon as an improvised non-reach weapon MAY be a possibility, but armor spikes should just be automatic IMHO)
Have him enlarge (or just have non-weapon reach somehow) coupled with "Come and Get Me" (rage power, APG) and "Punishing Kick" (feat/monk power, APG) so that when a PC comes up and attacks he gets kicked away before swinging. Good Dex with Combat Reflexes keeps melee PCs away, and he can kick them if their reaches match. Only do this to avoid Full Attacks as uses are limited, BUT he can still just hit them with his main attack too (and darn hard I'd think).
Also, Snake Fang (right snake feat?, UC) lets him unarmed strike when they miss him. This attack won't do much, so add some carrier damage/effect. This helps his action economy a bit.
Other feats that let him knock PCs around will help his action economy, if they constantly have to get up or move.
Invulnerable Rager will help somewhat, unless Uncanny Dodge helps more vs. party composition.
Superstitious would be valuable for the save boost and add maybe some anti-casting/anti-concentration rage powers too.
If he rage cycles, get the feat that lets him AoO just for entering a threatened square (1/rage, so cycle needed), and maybe that rage power (APG?) that lets him bullrush people away with an attack.
Also, Blindfight, as he'll likely get blinded or attacked by somebody Invisible. (It'll help his Blindsense be effective. Depending on your party you may want Improved Blindfight.)
And you may want Anti-Magic Field just to alter everything. This will nullify a lot of one-round kills, and with a build to go with AMF, he can thrive more than they. (Pin down arcane caster in corner and ouch.)
Buy some inherent bonuses, adamantine weapons, and sunder w/ breaker barb. (Barb 5/Sorcerer 5/DD 10 could cast it)
Or skip the DD and make him half-dragon so he can actually get all these rage feats and at maximum for those dependent on level. (Do you have any plans for him casting?)
And, if he has wings, Flyby Attack with Greater Vital Strike (while Enlarged) gives (w/greatsword) 12d6+1.5 Str+magic) which isn't much at CR 20+ BUT keeps those Full Attacks off of him.
Wait, he'd die too fast to ranged so...
Reach Weapon/Anti-Magic Field/GVS/nonmagical wings/Flyby Attack/Sunder their bows might work too, though he'd never outpace the healer and if likely to win, they'd just leave.
In the end, the party has so many resources at that level, that it's unlikely this monster can work very well RAW solo unless you trick it out even more or alter the environment heavily (burn damage every round? paralysis field?).
A roomful of 1/2 dragon Bugbear Barb 2/Warrior 16 (CR 14, believe it or not because of starting with racial HD) could give some nice Holy Word-proof thugs (21 HD) to distract parts of the party while he kicks ass on the others. (Str. 36+ possible with BAB +20 makes them worthwhile minions. They could use the Flyby/GVS trick, keeping them spread out to resist spells that much better. Of course, in a large space, a summoned Elder Elemental's going to come kick their asses.)
When the thugs die (die in a confused blaze of self-destruction/weird/wail), then BBEG could transform into a CR 20 dragon (as separate XP) now that he "has room".
So many tactics possible by your 20th level PCs that it's nearly impossible to advise without knowing their standard tactics/build and whether they come prepped or surprised, etc., so I'll leave it at this (as it was fun, but it's time to go, as there are too many counter-maneuvers to the above to address.)