Elfman's only feat in the series has been high durability. Arguing he wasn't defeated by Madmole doesn't mean much to me given he also doesn't have any offensive feats, and he was with Mira, who would protect him. We also don't know if the attack Lucy hit the Strauss siblings with is her strongest attack, so she could still have something more powerful. Imo, if they fought one vs one, Lucy would probably beat him up as long as her stamina lasts, since she has better speed feats and she would have 3 spirits for backup. If she has a stronger star dress mix for one vs one combat, or she can pull out enough attacks with her spirits she takes it imo, and if not then Elfman just pulls a Homer Simpson, and waits for Lucy to tire herself out before finishing her off like against Bacchus. I don't see any reason Elfman would take this low difficulty, and nor would Lucy.