To unlock the Grind 'n' Bear It trophy in Borderlands: The Pre-Sequel, you will first need to complete the fifth story mission "A New Direction".
Once you complete "A New Direction", you will receive a new mission from Janey Spriggs called "Grinders". When you complete that, return to Concordia and build the Grinder. Put three guns of the same colour in it, or use the three white weapons that Janey provides you with. Let the weapons grind and the trophy will unlock.