If you're trying to challenge a 400k Isabelle with a 300k Eremia, then that's the problem. Matchup advantages can only overcome so large of a power discrepancy.

But at the top tier, there aren't any particularly strong Isabelle characters. Even now while she's boosted, I don't think I've ever seen an Isabelle over 450k power. Having gone undefeated in arena the last three consecutive weeks, I see a lot of characters over 450k. I have five such characters myself, and at least three others that I could push over 450k by swapping around my best gear. And I see quite a few characters over 500k, even.