You're running out of time
Students can cram grammar and vocabulary in hours. Improving pronunciation takes years
I have taught English conversation in Korea for 23 years. The hardest thing to improve is pronunciation.
Pronunciation is about muscles, and muscles require repetitive practice. Extensive repetitive practice.
You're in a race against time.
Every minute students get older, improving pronunciation takes more time and effort.
If you don't believe me, think about Jackie Chan and Arnold Schwarzenneger.