Hellfire wyrm - First thoughts is that its attack routine is utterly worthless at its level, averaging just 40 damage, and at a paltry +30 to-hit (at a point where PCs average 45-50 AC). So in the end, you get a dragon-shaped spellcaster of sorts. At this point, low-lv SLAs like charm, suggestion, hold person, unhallow, desecrate, misdirection, pyrotechnics and wall of fire just don't cut it, due to the PCs' likely high saves and resistances.
Not only that, but its caster lv of 20 means that its blasphemy/dictum SLAs won't affect PCs of 21th lv or higher, leaving improved invisibility, firestorm, greater dispelling, teleport and unholy aura as its more useful abilities. So it is essentially relegated to the role of an aerial bomber. However, its key attacks can easily by negated (freedom of movement counters crush, heroes' feast makes frightful presence moot, resistances/immunitues negate firestorm/breath weapon). So you need quickened dispels to get rid of the players' buffs first.
1) Advance its HD to 25 to increase its size to gargantuan.
2) This improves its physical stats (str38, dex10, con31), hp 412, AC38.
3) Attack routine (+35/+35/+35 with improved multiattack)
4) Damage: Rapidstrike/rend (draconomicon), improved natural attack
5) Crush affects up to medium sized targets.
6) Replace feats with quicken sla: greater dispel, snatch, power attack, rend, flyby attack, rest your choice (suggest multiattack/rapidstrike/INA: Claws).
7) New cr is likely closer to 18-20, ensuring its blasphemy/frightful presence remains relevant, using the chichemic (ELH) as a yardstick.
Let me know what you all think?