(Put in spoilers just in case.) You get it by completing Nagi's affinity missions. As for how it's ruining it- I'm assuming it's because Overdrive, Offensive Stance and Blossom Dance can clear out one or two HP bars at once, at least from what I've seen. Given that it's a popular build, too, that means that a lot of RP is being taken very quickly. At least, that's what I think it means.