Defensive stats barely affect a girl's power rating, but they sure do affect how strong she is in battle. A girl at 300k power that focused heavily on defensive stats to get there will handily destroy the same girl at 300k power that focused purely on offensive stats and ignored defensive ones. Even so, when you have a choice of which stat to get in a given slot (such as stats on weapons), you nearly always want an offensive stat. It might be that the offensive stat is weighted 10 times as much in the power rating, but only twice as powerful in actual combat--but still more powerful than the defensive stat in combat.

Your other problem is that you're using Claire and Ortylia in arena. They're both terrible at arena, unless Ortylia gets her Tempting Chaos skill to take effect, which is unreliable. Claire's only real use was healing in raids and tower of trials, and Ortylia is much better in both of those situations.