As of this posting the effects of inflicting stat damage to a mob are sub-par at best. For STR, DEX, and CON, the mob becomes stunned for a short time, then seems to become immune to further stat damage. It doesn't seem that damage to INT, WIS, or CHA has any effect whatsoever!
Two solutions -- the Easy Way and the Hard Way
The Easy Way
When a mob's stat is drained to 0, the standard effects occur. In addition, any stat damage to the mob against a stat that was drained to 0 would instead deal 10 untyped damage per point drained. This damage would completely ignore ALL forms of damage reduction.
The Hard Way
STRENGTH: When a mob's strength is drained to 0, they attack at a 50% reduction in speed and they become immune to strength damage until their stat is brought above 0.
DEXTERITY: When a mob's dexterity is drained to 0, they lose all evasion and improved evasion capability, and move at -75% speed. They become immune to dexterity damage until their stat is brought above 0.
CONSTITUTION: When a mob's constitution is drained to 0, their maximum Hit Points drop by 50% and automatically fail fortitude checks on a roll of 1-3. They also become immune to constitution damage until their stat is brought above 0.
INTELLIGENCE: When a mob's intelligence is drained to 0, they lose the ability to cast spells based on INT. They gain immunity to stat damage to intelligence until that stat is brought above 0.
WISDOM: When a mob's wisdom is drained to 0, they lose the ability to cast spells based on WIS. They also become easily controlled, automatically failing saves vs. charm. They gain immunity to stat damage to wisdom until that stat is brought above 0.
CHARISMA: When a mob's charisma is drained to 0, they lose the ability to cast spells based on CHA. Further damage to CHA deals 10 points of untyped damage per point drained.