To clarify what's going on. Thunder Slash is bugged, and has an accuracy of 0%, so it will almost always miss unless Steiner and playable Beatrix are way overleveled from the enemy they are using on. It also does fractional damage so it will never work on bosses. Despite all of the remasters/ports, it's never been fixed.

Oddly enough, the boss version of Beatrix doesn't use a bugged version of the attack, so that version is the only one that works.