How to EASILY Unlock the "Knowledge Is Power" Achievement | Dead Cells Return to Castlevania DLC 

If you repeatedly hit a closed, non-breakable door with the Bible you'll be able to generate six projectiles at once. The achievement "Knowledge is Power" requires you to have five, so this will definitely do the trick. Instead of trying to get the Bibles spinning while fighting a boss, just use a door. The doors you can reliably do this on are the ones in the Bank, the ones that you need to press a switch to open, or the one after Guillian that stays locked when you need to choose mutations. Easy!
Please note, this may NOT work on mobile. See the pinned comment below!