You will fight Killabilly, a boss at the end of Stage 6.
During the second phase of this boss battle, you will be in an open rooftop with Killabilly in the background. He will constantly be slamming both his hands onto the ground before and after each of his attacks. As you might have guessed it, aside from shooting this ugly mug with your Blaster, attacking his hands is the only way to damage him. Each time you damage his hands, you will cut off a finger, which will magically regenerate back a few moments later for you to cut again. You will have to cut 20 of his fingers to earn the trophy, which will more than likely come naturally.
If for some reason, you couldn't cut 20 of his fingers, you will earn it during Ranking Mode. This trophy is cumulative, so you don't need to cut them all in one battle.