So i did a MB attack and defeated Chienne with
You're browsing the GameFAQs Message Boards as a guest. Sign Up for free (or Log In if you already have an account) to be able to post messages, change how messages are displayed, and view media in posts.
Pretty much any condition that requires you to "defeat/force retreat X enemy with Y character", you need to have the appropriate character initiate the attack. Who reduces the enemy to 0 (and thus earns the kill) is irrelevant.