Well, if you limit it to 32-point buy and the suggested 760,000gp worth of items that the DMG suggests, it becomes slightly reasonable. But not hard.
Brunhilda
Female Elf 19th level rogue/1st level sorcerer
Str 12 . Dex 24 . Con 6 . Int 13 . Wis 10 . Cha 12 After stat increases, before magic items.
HP 32
Feats: Point-blank shot, Weapon Focus (composite longbow), dodge, mobility, shot on the run, rapid shot, alertness.
Abilities: Sneak attack +10d6, can cast true strike and something else.
Items:
Ring of Improved Invisibility (or a lot of potions or a lot of scrolls)
Cloak of Flying, or the equivalent.
+5 Magical-beast Bane Icy-burst Mighty Composite Longbow (total enchantment +9)
Quiver of Ehlonna filled with 60 +5 shocking-burst flaming-burst arrows (total enchantment +9)
Girdle of Strength +6
Gloves of Dexterity +6
Ranged Attack Bonus: +32
You'll be invisible, and gain a +4 bonus to hit, so you'll always hit with your first shot, dealing 1d8+10d6 (sneak) +2d6 (bane) +7 (enchantment) + 1 (point blank shot) +4 (strength) +1d6 +1d6 +1d6 (frost, flaming, icy). That's 1d8+15d6+8, averaging 66 damage per hit. After regeneration, that's still 26 points of damage per round, so you'll eventually whittle it down to death. Since you're flying above its head, and invisible, you'll never be hit. You win.