I think Luna's attitude makes sense considering that canonically, she refused all of Iveiru's temptations after signing the contract and ended up killing him. Meryl went after a much weaker target with Hayato and even then only had questionable results (Since the dev seems to have backtracked on Meryl successfully corrupting Hayato in the first game to make the relationship between them more ambiguous for the sequel).
2 points I would like to highlight here:
1- while Luna repelled Iveru at the end his influence on her seems to linger on as mentioned by the developer, which led us to believe that the contract he made with her has progressed to some extent just like Hayato has with Meryl, speaking of Meryl, she didn't forcefully seduced Hayato (at least not at beginning) as she wanted him to desire her from his own free will and she played him in the palm of hand easily throughout the game while we as players has little to no choice other than seeing him being toyed by her unlike Celia who had the choice to resist Sadi's advances, not to mention that it only took Meryl to completely dominate Hayato is him having sex with her just 1 time while Kurone and Iveru have no such feat at all which make her a terrifying enemy, I think the only reason managed to resist her at the end when she stopped playing nice is the bond between him and Celia. Realistically speaking, Hayato and Celia couldn't have won this, Sadi and Meryl has successfully manipulated and swayed countless people before and they had prep time as well, it is baffling that with all of this on their side they couldn't take down Celia and Hayato, I'd like to think that they were overconfident in thier victory and simply wanted to slowly corrupt the 2 innocent youngsters which led to their defeat.
2- if Leo didn't resist Kurone's temptation at the end, Luna would've ended up as Iveru slave just as seen in the bad ending, so it's not like she held her own very well.
She underestimated Iveru which is how he got an in.
You see my friend, this IS the problem, Luna made a big mistake by underestimating Iveru and it put her in a very vulnerable position, and she almost got enslaved by him for real, so what did she learn from this experience as seen in her interaction with Meryl? nothing, she simply looks down on her calling her a small fry while she forgot how she was almost taken down by a similar opponent last time, it's one thing that she made a mistake, but repeating it over again is a plain stupidity.