Using just the monster manual you could be up against a:
GREAT WYRM HALF-CELESTIAL GOLD DRAGON
Colossal Outsider (Fire, Good, Lawful)
Hit Dice: 71d12+923 (1384 hp)
Initiative: +7
Speed: 60 ft, fly 250 ft (clumsy), swim 60 ft
AC: 46 (+3 Dex, -8 size, +41 natural)
Attacks: Bite +81 melee, 2 claws +76 melee, 2 wings +76 melee, tail slap +76 melee
Damage: Bite 4d8+18, claw 4d6+9, wing 2d8+9, tail slap 4d6+27
Face/Reach: 40 ft by 80 ft / 15 ft
Special Attacks: Breath weapon, spell-like abilities, luck bonus, detect gems, half-celestial spell-like abilities, frightful presence
Special Qualities: +4 save vs. poison, water breathing, immunities, DR 20/+3, blindsight, keen senses, SR 33
Saves: Fort +50, Ref +40, Will +49
Abilities: Str 47, Dex 16, Con 37, Int 35, Wis 34, Cha 33
Skills: Bluff +82, Concentration +84, Diplomacy +82, Escape Artist +74, Jump +89, Knowledge (arcana, architecture, geography, history, nature, nobility, planes, religion, plus any 3) +83, Listen +85, Scry +83, Search +83, Sense Motive +83, Spellcraft +83, Spot +85
Feats: Hover, Flyby Attack, Power Attack, Cleave, Snatch, Improved Initiative, Spell Focus (Evocation), Alertness, Wingover, Quicken Spell-Like Ability, Sunder, 7 more
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Challenge Rating: 28
Alignment: Always lawful good
SA–Breath Weapon (Su): Cone of fire, 70 feet long, every 1d4 rounds; damage 24d10, Reflex half DC 58. This dragon can use a cone of weakening gas instead, forcing subjects who fail a Fortitude save to take 12 points of temporary Strength damage.
SA–Frightful Presence (Ex): Dragons can unsettle foes with their mere presence. This ability takes effect automatically (radius 360 ft) whenever the dragon attacks, charges, or flies overhead. A potentially affected creature (one with less than 71 HD) that succeeds at a Will save (DC 56) remains immune to that dragon's frightful presence for one day. On a failure, creatures with 4 or fewer HD become panicked for 4d6 rounds and those with more than 5 HD become shaken for 4d6 rounds. Dragons ignore the frightful presence of other dragons.
SA–Spell-Like Abilities: 3/day - polymorph self and bless; 1/day - geas/quest, sunburst, foresight.
SA–Luck Bonus (Sp): Once per day the dragon can touch one gem (usually embedded in the dragon's hide) and enchant it to bring good luck. As long as the dragon carries the gem, it and every good creature in a 120 foot radius receives a +1 luck bonus as per stone of good luck. If the dragon gives the gem to another creature, only the bearer gets the bonus. The enchantment lasts 1d3+36 hours but ends if the gem is destroyed.
SA–Detect Gems (Sp): 3/day, the dragon can use this ability. Similar to detect magic, this spell finds only gems. Studying for one round reveals presence or absence of gems, 2 rounds reveals exact number of gems, and 3 rounds reveals exact location, type, and value.
SA–Half-Celestial Spell-Like Abilities: 3/day-protection from evil, holy aura; 1/day-bless, aid, detect evil, cure serious wounds, neutralize poison, holy smite, remove disease, dispel evil, holy word, hallow, symbol, summon monster IX (celestials only), resurrection.
SQ–Immunities (Ex): Immune to acid, cold, disease, electricity, sleep, paralysis.
SQ–Water Breathing (Ex): This dragon can breathe underwater indefinitely and can freely use its breath weapons, spells, and other abilities while submerged.
SQ–Blindsight (Ex): Dragons can ascertain creatures by nonvisual means (mostly hearing and scent, but also by noticing vibrations and other environmental clues); this dragon's range is 360 feet.
SQ–Keen Senses (Ex): Dragons can see four times as well as humans in low-light conditions and twice as well in normal light. They also have darkvision (range 1200 feet).
Spells Known (cast 6/9/9/9/8/8/8/8/7/5; as a level 19 sorcerer):
0–detect magic, resistance, detect poison, read magic, dancing lights, ghost sound, mage hand, open/close, mending; 1–endure elements, mage armor, divine favor, cure light wounds, shield; 2–bull's strength, cat's grace, blur, endurance, fog cloud; 3–haste, dispel magic, protection from elements, slow; 4–spell immunity, dimension door, improved invisibility, detect scrying; 5–cloudkill, righteous might, dominate person, teleport; 6–heal, greater dispelling, harm; 7–spell turning, destruction, greater scrying; 8–mass charm, maze, protection from spells; 9–miracle, foresight.
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And that's if it's using NOTHING from it's horde. Add in some precast spells like mage armor, divine favor, shield, bull's strength, cat's grace, blur, endurance, haste, improved invisibility, righteous might, spell turning, and protection from spells and it's even more ugly.
The PHB has nothing that can compete with that. The DMG has nothing that can compete with that. Maybe a sphere of annihilation, but you'd just end up getting annihilated by it because you now don't have access to spells like fox's cunning etc.
Dead PC!
