Victory is Mine is a music track that is unlocked in a Champion's Challenge fight. It is unlocked by any fight which takes place in the central arena, including rematch fights against the Leprechaun Champion.
When you fight the leprechaun, the fight is not in the main arena. Since the Champions Challenge resets weekly, you can fight the leprechaun in practice first, then use a D&D token (weekly) to reset the leprechaun fight (or wait for weekly reset) so he appears in the main arena when you rematch him, which then will unlock the music track.
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