If you go by lore maybe she could win this fight since she was confident enough in her battle mode to go up against Mirajane but she got one shotted before she could use a single technique.
While she had such certainty, Jenny proposed Mirajane a bet, which would have involved the loser posing nude for the Sorcerer Magazine; something which hinted at a certain degree of malice on her side.
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 This was shown again when she and the others were at the Ryuzetsu Land and she takes off Mirajane's bikini top and bottom for revenge for her loss the other day.
 Similarly, she seems to be very confident in her own abilities, particularly in her appearance.
Let's continue with the trivia.