Pinball is a "Note Power Up" unlocked via Blitz Cred.
Pinball notes will show up on the field during play, as a purple note. Hit them once to activate Pinball mode, in which a flying metal ball will fly up the screen and back down. You must keep the pinball in play by blocking it's exit off the bottom of the screen with your vehicle.
If you are having difficulty controlling the pinball, ignore the notes and focus on keeping the ball in play.
In addition, try entering the match with "Bandmate" and "Pinball" selected. Activating the "Bandmate" power up will create a second vehicle which can and will block the pinball.
The achievement will unlock immediately when the Pinball has been on the field for 20 seconds. This must be done using a single pinball.