VERDICT: Nightjar Slash isn't specialized towards any specific use, but it is good because it's essentially just a better version of a regular sprint attack. This makes it broadly useful against every boss, but not especially useful against any of them, with one exception - I found it to be VERY strong vs. Owl, because he specifically jumps away from you a lot and has a lot of moves that create distance between you and him, and Nightjar Slash allows you to close that distance very fast. Owl also has a lot of moves that are strong, but very few of them have super armor, meaning most of them can be interrupted with just a regular attack. By using Nightjar Slash heavily during his fights, I found myself very frequently interrupting a lot of his attacks. So I'd definitely recommend it against Owl. You can also use it to counter perilous sweep attacks, but doing so doesn't give you the bonus Posture damage that a simple jump counter does and it can also be harder to time against certain attacks, so it's objectively worse for that specific purpose.
VERDICT:
Nightjar Slash isn't specialized towards any specific use, but it is good because it's essentially just a better version of a regular sprint attack. This makes it broadly useful against every boss, but not especially useful against any of them, with one exception - I found it to be VERY strong vs. Owl, because he specifically jumps away from you a lot and has a lot of moves that create distance between you and him, and Nightjar Slash allows you to close that distance very fast. Owl also has a lot of moves that are strong, but very few of them have super armor, meaning most of them can be interrupted with just a regular attack. By using Nightjar Slash heavily during his fights, I found myself very frequently interrupting a lot of his attacks. So I'd definitely recommend it against Owl. You can also use it to counter perilous sweep attacks, but doing so doesn't give you the bonus Posture damage that a simple jump counter does and it can also be harder to time against certain attacks, so it's objectively worse for that specific purpose.