殫精竭慮

1. • to exhaust one's thoughts and ingenuity (idiom) to think sth through thoroughly
2. • to rack one's brains
3. • to leave no stone unturned

Character by character

• 殫(dān)entirely
• 精(jīng)essence
• 竭(jié)exhaust
• 慮(lǜ)think over