Finger Pickin' Good Trophy in Rock of the Dead
Finger Pickin' Good
Complete 20 Challenges.
How to unlock Finger Pickin' Good
These will show up in as roll-off, or enemy challenges in certain areas during a level. You just need to complete any 20 challenges in the campaign. It is likely you'll get these naturally considering the amount of challenges in each level. The most common challenges are humans trying to get to safety from swarms of enemies in which you will have a limited amount of time to appropriately hit the notes shown on the screen otherwise they will die and the challenge will be considered failed.