This is in Episode 2: Get Tannen.
In 1986, when you see 3 Tannen brothers appear and try to gang up on you.
Choose a topic to ask about.
Then choose Dodge >
This will send Marty over on top of a crate in front of the garage.
Choose another topic to ask about.
Then choose Jump.
A Tannen will knock out the crate Marty's standing on over to the other side.
Choose another topic to ask about.
Then choose Dodge >
This will send Marty over on top of the crate again.
Choose another topic to ask about.
Then choose Jump.
A Tannen will knock out the crate, which will hit Biff then fall over to the porch.
Choose another topic to ask about.
Then choose Dodge >
This will send Marty back to the front porch and onto the crate again.
Choose another topic to ask about.
Biff and bros will then talk about their connection to JJ Valenti.
Choose Duck to knock all 3 Tannen brothers out.
The trophy will pop after Doc and Marty escape.
Congrats!