The correct sentences are The previous valedictorian passed this grueling syntax class purely by dint of sheer perseverance and The reading list was drastically shortened at the behest of the university dean.

Idiom Breakdown:

• By dint of means "by means of" or "as a result of."

• At the behest of means "under the command or request of."

• The phrase On lieu of is incorrect; the proper idiom is in lieu of (meaning "instead of").

• The phrase for the detriment of is incorrect; the proper idiom is to the detriment of (meaning "causing harm or damage to").