#6. Any 5th-level sorcerer or wizard who succeeds at a caster level check when using a scroll of empowered disintegrate scribed at 20th level, confirms a critical with the attack roll, whose opponent fails the save, and rolls maximum damage.
An opponent who fails his save against a 20th-level empowered disintegrate takes a maximum of 40 x 6 x 1.5 = 360 points of damage, or 720 points on a critical hit.
EDIT #1: A scroll of empowered disintegrate scribed at 20th level costs 8 x 20 x 25 gp = 4,000 gp, less than half the standard wealth of a 5th-level character (9,000 gp).
EDIT #2: Alternatively, if the damage does not have to be done in a single round, it could be a scroll of repeat spell disintegrate. The Repeat Spell feat is from Complete Arcane. If the opponent does not move more than 30 feet from his position, fails his save against both spells, and both spells critical, the opponent takes 40 x 6 x 2 x 2 = 960 points of damage over two rounds. The scroll costs 9 x 20 x 25 gp = 4,500 gp, still within the means of a 5th-level character.
EDIT #3: A warmage can also pull off this trick. A character using a scroll still needs to have the minimum ability score to cast the spell (or emulate the ability score with Use Magic Device), but this can be done by putting an 18 into Intelligence (for a wizard) or Charisma (for a sorcerer or warmage) and using the 4th-level ability score increase to up it to 19.
EDIT #4: Against an opponent who is an evil outsider, it is still possible to get 960 points of damage in a single round, but you need a good-aligned character (or emulate the alignment with Use Magic Device), and a scroll of empowered purified disintegrate. The Purify Spell feat is from the Book of Exalted Deeds, and increases the damage dealt by a spell to an evil outsider by one die (d6 to d8 in this case) and increases the level of the spell by +1. The spell will thus do 40 x 8 x 1.5 x 2 = 960 points of damage.
EDIT #5: A 32-point buy character could look like: Str 10, Dex 12, Con 14, Int 18, Wis 10 Cha 10. Simply swop Int and Cha for sorcerers and warmages.