By "bullet" they mean the weird bone like projectile that the Tricorne Ooze enemies shoot at you. These seem to start to appear in Chapter 3-1 on Normal Mode. It takes a bit of practice, but after a few attempts, you should be able to get this.
A few tips:
- Kill all other enemies neaeby
- Make sure there are no obstructions
- Stand still
- Swing your knife
when the bone is about a foot or two away