I have a tendency to correct people when they tend to say things that are either shallow without depth, or misinterpret information. It is not exclusive to crossover battles, they can be done with any fictional battle, there is no rule restraining that from one another. No Acnologià is stronger than Irene as he scales to Phoenix as it can destroy the world, Irene’s Compression of Fiore would be somewhat comparable, her Dragon Sage powers would be stronger though. They did damage Madmole though, and Skullion, so that supports Natsu’s status, he was doing nothing by beating on Wraith with no magic spells to power him up When he made that statement, even Makarov is confirmed stronger, so the Spriggans are still at a level above, we do not see Mirajane one shooting as she did not recently transformed into her form, and she was not holding back at the time when there are no people around.