Only a fully trained Jedi knight, with the Force as his ally, will conquer Vader and his emperor.
9 seconds sound clip from the Star Wars: Episode V - The Empire Strikes Back movie soundboard.
You can hear this line at 01:24:06 in the Blu-ray version of the movie.
Quote context
[...]
- That is why your friends are made to suffer.
- Luke, I don't want to lose you to the emperor the way I lost Vader.
- Stopped they must be. On this all depends.
- Only a fully trained Jedi knight, with the Force as his ally, will conquer Vader and his emperor.
- If you honor what they fight for... yes.
- If you choose to face Vader, you will do it alone. I cannot interfere.
[...]